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PRIKCIPLES  OF  HISTOLOGY 


A.  H.  TUTIXE 


THHHi 


THE  PRINCIPLES  OF  HISTOLOGY 


DHSCKIPTIVIC  AND  PRACTICAL 


BOOK  I. 


DESCRIPTIVE    HISTOLOGY. 


BY 


ALBERT  H.  TUTTLE. 


PUBLISHED    BY   ANDERSON    BROS. 

UNrV'ERSlTY  OF  VIRGINIA. 

1898. 


Entered  according  to  Act  of  Congress  by  Albert  H.  Tuttle, 
University  o£  Virginia,  J  898, 


Press  of  Prout  the  Printer, 

Charlottesville,  Va. 


ft 


^ 


PREFACE. 

This  manual  represents  an  effort  to  state  the  most  im- 
])ortant  facts  of  descriptive  histology  in  a  manner  adapt- 
ed to  the  wants  of  my  own  classes  of  students,  both  aca- 
demic and  medical.  It  lays  no  claim  to  originality  save 
in  the  arrangement  and  mode  of  presentation,  and  ac- 
knowledgement is  here  made  of  the  extensive  use  of  the 
])est  modern  treatises  and  monographs  accessible  in  its 
preparation.  In  the  portions  dealing  with  the  nervous 
system  I  am  under  special  obligations  to  the  writings 
of  Cajal  and  Van  Gehucten  among  others.  I  am  in- 
debted to  Dr.  Lyman  J.  Skeen  for  frequent  and  valuable 
aid  in  the  preparation  of  the  book. 

This  volume  will  be  followed  by  a  second,  now  in  course 
of  preparation,  dealing  with  Practical  Histology. 

ALBERT  H.  TUTTLE. 
University  of  Virginia,  Mav,  1898. 


CONTENTS    OF   BOOK    1. 


PART  1:    THE  CELL  AND  THE  TISSUES. 


CHAPTER  L 

Page 

Introductory:  Definitions:  The  Cell,    -    -      14—  IG 


CHAPTER  n. 
The  Epithelia :  Endothelium:  Epithelioid 
AND  Endothelioid  Structures, 17—  22 

CHAPTER  HL 
The  Cartilage  Group:  Cartilages,      -    -    -      23—  30 

CHAPTER   IV. 
The  Fibrous  Tissues:  Areolar  Tissue :  Adi- 
pose Tissue:  Retiform  Tissue  :  Membranes: 
Tendons, 31—  4-6 

CHAPTER  V. 
The  Lamellated  Tissues  :  Corneal  Tissue  : 
Osseous   Tissue  :    Periosteum  :     Marrow  : 
Bones, 47—  58 

CHAPTER  VL 
Ossification  :  of  Membranes  ;  in  Cartilage,      59—  72 


YI  CONTENTS. 

CHAPTER  VII. 
The  Blood,  - 73—  82 

CHAPTER   VIII. 
The  Contractile  Tissues:  Muscles,    -    -    -      83 —  90 

•      CHAPTER  IX. 

The  Small  Vessels:  Bloodvessels;  Lym- 
phatics: Serous  Cavities,   - 91—  98 

CHAPTER  X. 

The  Nervous  Tissues:  Nerve  Fibres;  Cor- 
puscles :  Terminals  :  Nerves  ;  Ganglia  : 
Neuroglia, ._----_-      99 — ]14 

CHAPTER  XI. 

The   Structure   of   the   Cell:  Nuclear 
Division,     --------.-----    115—123 


PART  II:  HISTOLOGICAL  ANATOMY. 


CHAPTER  XII. 

Introductory:  The  Embryonic  Tissue  Lay- 
ers:   Systems  of  Organs,    -------    127—134 


CONTENTS.  VII 

CHAI'TBK   XIII. 
Thk  Skin  and  Appendages:  Epidermis:  Dermal 
Glands:    Hairs:  Nails, 13r)— 152 

CHAPTER    XIV. 
The   Mouth  and  its  Contents:   Glands  of 
THE  M-wcous  AND  Serous  Type;s:  Salivary- 
Glands:  the  Teeth:  the  Tongue,      -    -    -    153—170 

CHAPTER  XV. 
The  Alimentary  Canal:  Component  Stra- 
ta: The  Pharynx:  The  Oesophagus:  The 
Stomach  :  The  Duodenum  :  The  Small  In- 
testine: The  Colon:  The  Rectum:  The 
Pancreas:  The  Liver,     -    -    - 153—190 

CHAPTER   XVI. 
The  Respiratory  Apparatus:  The  Trachea: 
The  Bronchi.:  The  Lungs:  The  Pulmon- 
ary Blood  Supply,    - 191—202 


CHAPTER  XVn. 

The   Urinary    Organs;   The  Kidneys:    The 
Bladder:  The  Urethra. 203 — 216 


CHAPTER  XVin. 
The  Male  Reproductive  Organs:  The  Scro- 
tum ;  The  Testes:  The  Spermatic  Ducts 
and  Seminal  Vesicles:  The  Male  Ure- 
thra :  The  Penis  :  The  Prostate  :  Cow- 
per's  Glands, 217—232 


YIII  CONTENTS. 

CHAPTER  XIX. 

The  Female  Reproductive  Organs:  The 
Ovaries:  The  Oviducts:  The  Uterus:  The 
Vagina:  The  Vulva:  The  Mammary 
Glands:  The  Homologies  of  the  Urino- 
Gexital  Organs, __...  233—258 


CHAPTER  XX. 

The  Vascular  System;  The  Arteries;  The 
Heart;  TheVeins:  The  Lymphatic  Trunks; 
The  Serous  Membranes:  The  Synovial 
Membranes, .-    259—268 


CHAPTER  XXI. 

The  Ductless  Bodies:  Lymphatic  Nodules 
AND  Nodes:  The  Spleen:  The  Thymus:  The 
Thyroid:  The  Parathyroids,  Carotid 
Glands,  AND  Coccygeal  Gland:  The  Adre- 
nal Bodies:  The  Pituitary  Body:  The 
Pineal  Body, -    269—286 


CHAPTER  XXII. 

The  Nervous  System  :  The  Meninges  :  The 
Spinal  Cokd:  The  Cerebellar  Cortex: 
The  Cerebral  Cortex, 287—320 


CHAPTER  XXIII. 

The  Organs  of  Special  Sense;  The  Taste 
Buds;  The  Organs  of  Smell;  The  Eye; 
The  Ear,   -    - 321—360 


BOOK  I. 

DESCRIPTIVE   HISTOLOGY, 


PART    I. 


THE  CELL  AND  THE  TLSSUES. 


\-\-{^yt<YJ^v~, 


CHAPTER  I. 
INTRODUCTORY. 


Histology  is  the  science  that  treats  of  tissues;  their 
structure,  their  components,  their  development  luul  modi- 
fication, and  their  arrangement  to  form  the  organs  of  the 
body.  We  may  distinguish  between  Animal  and  Vege- 
table and  between  Normal  and  Pathological  Histology, 
as  well  as  between  Human  and  Comparative  Histology ; 
expressions  which  define  themselves.  As  the  term  Histol- 
ogy is  most  commonly  used  in  medical  literature  it  signi- 
fies the  normal  histology  of  man. 

In  its  most  limited  sense  the  term  Histology  is  applied 
to  the  description  of  the  tissues  alone.  This  description  is 
also  sometimes  called  General  Histology.  The  discussion 
of  the  arrangement  of  the  tissues  to  form  the  organs  of 
the  body  is  distinguished  as  Histological  Anatomy ;  or, 
(less  appropriately)  Physiological  Anatomy. 

A  Tissue  may  be  defined  as  a  mass  of  similar  structural 
elements  (cells  or  cell-derivatives)  having  similar  prop- 
erties and  functions;  together  with  such  substances  as 
are  characteristicalh'^  present  between  the  elements  (and 
are,  as  a  rule,  formed  by  their  action),  serving  to  unite 
them  together. 


14  PART  I.      THE    TISSUES. 

This  definition,  while  true  of  the  great  majority  of  tis- 
sues, cannot  be  universally  applied  in  a  literal  sense. 
There  are  some  tissues  (e.  g.,  adenoid  tissue)  regularly 
involving  more  than  one  kind  of  element.  Such  may  with 
propriety  be  called  Compound  Tissues. 

The  combination  of  tissues  gives  rise  to  organs  of  defi- 
nite structure  and  function;  and,  conversely,  every  organ 
is  ultimately  resolvable  into  its  component  tissues. 
There  are,  however,  certain  tissue-aggregates  which,  while 
themselves  entitled  to  be  regarded  as  organs,  sustain  a 
similar  relation  to  the  structure  of  the  larger  and  more 
complex  bodies  to  which  that  term  is  applied  as  do  the 
tissues  themselves.  Among  such  are  the  smaller  blood  and 
lymph  vessels,  small  glands,  adenoid  masses,  etc.  The  con- 
sideration of  these  compound  factors  of  structure  may 
properly  accompanj'-  that  of  the  tissues. 

There  also  occur  in  the  body  substances  (notably  the 
blood)  which,  while  they  lack  that  coherence  properly  as- 
sociated with  the  idea  of  a  tissue,  contain  cellular  or  cor- 
puscular elements  analogous  to  those  forming  the  basis 
of  tissue-structure.  While  they  can  hardly  be  classed  as 
tissues,  their  study  is  clearly  within  the  province  of  His- 
tology. 

The  cells  (or,  more  strictly,  the  corpuscles)  to  which 
reference  has  been  made,  although  of  exceedingly  various 
form  and  size,  are  fundamentally  similar  in  that  each  is  a 
mass  of  (more  or  less  modified)  protoplasm  provided  for  a 


CHAPTER    I.      INTRODUCTORY.  15 

portion  if  not  the  whole  of  its  life  with  a  nucleus.  In 
some  cases  the  outer  layer  of  the  protoplasm  ^.^ives  rise  to 
or  is  modified  to  form  a  more  or  less  well-defined  mem- 
brane or  w^all  of  varying  composition,  thus  forming  a  true 
cell ;  but  the  constant  presence  of  a  cell-wall  of  definite 
and  approximately  uniform  composition  characteristic  of 
the  tissue  elements  of  plants  cannot  be  affirmed  of  those  ot 
animals,  the  converse  being  more  generally  true. 

The  study  of  cells  as  living  beings,  their  internal  struc- 
ture, activities  and  life-histories,  or  Cytology,  is  an  im- 
portant and  rapidly  growing  branch  of  Biology.  We  are 
here  concerned  with  them  as  components  of  the  tissues 
and  with  their  structure  and  activities  as  related  thereto. 
Such  discussion  thereof  as  this  implies,  while  it  logically 
precedes  an  account  of  the  tissues,  is  on  the  whole  best 
deferred  until  some  practical  familiarity  with  them  and 
with  the  elements  that  compose  them  has  been   acquired. 

It  is  sufficient  at  this  time  to  state  that  the  term  proto- 
plasm is  applied  to  what  has  until  recently  been  regarded 
as  one  substance,  of  exceedingly  complex  chemical  compo- 
sition and  apparently  devoid  of  structure;  clear  or  slighth' 
granular  in  appearance  under  ordinary  powers  of  the 
microscope;  and  viscid  or  semi-solid  in  consistency:  im- 
proved methods  and  appliances  of  research  have,  how- 
ever, shown  that  it  is  neither  homogeneous  nor  structure- 
less. It  is  the  seat  of  the  processes  which  make  up  physi- 
cal life;  and  it  and  its  products  compose  the  living  body, 
which  in  its  earliest  stages  consists  of  a  number  of  appa- 
rentl}'  similar  minute    nucleated  masses  of  protoplasm 


16  PART    I.      THE    TISSUES. 

(embryonic  cells):  the  specialization  in  each  of  these  masses 
of  some  one  function,  and  the  associated  modification  in 
the  form  of  the  mass  and  of  its  solid  products,  if  any, 
give  rise  to  tissue-differentiation. 

The  nucleus  is  a  body  usually  spheroidal  in  form,  both 
physically  and  optically  denser  than  the  protoplasm  by 
which  it  is  surrounded.  It  is  jclearly  seen,  even  with 
ordinary  microscopes,  to  be  more  complex  in  structure 
than  the  latter :  a  nuclear  membrane,  a  more  or  less  defi- 
nite intranuclear  network,  and  occasional  granules,  the 
largest  of  which,  when  distinctly  spheroidal  in  form,  are 
known  as  nucleoli,  being  in  many  cases  readily  discerni- 
ble. During  cell-multiplication  the  nucleus  is  the  seat  of 
important  changes,  which  will  be  discussed  in  a  subse- 
quent chapter,  together  with  its  structure  and  that  of  the 
surrounding  protoplasm. 

While  we  may  recognize  a  large  number  of  tissue-ele- 
ments differing  from  each  other  in  form  and  size,  as,  of 
course,  in  function,  they  can  all  be  included  in  a  very  few 
primary  groups,  as  follows. 

Epithelial,  those  lining  or  investing  a  free  surface. 

Skeletal,  those  forming  an  investing,  supporting  or  pro- 
tective framework  for  an  organ  or  for  the  whole  body. 

Contractile  or  Muscular,  those  producing  by  their  com- 
bined action  definite  movements. 

Irritable  or  Nervous,  those  acting  as  reservoirs  of 
energy,  or  as  channels  for  its  discharge,  or  as  receivers  and 
distributors  of  stimuli. 

Reproductive  (modified  epithelial)  elements. 


CHAPTER    II.      THE    EIMTHELIA.  17 


CHAPTER   II. 
THE  EIMTHELIA. 

Epithelium  may  be  defined  as  a  continuous  layer  of 
cells  always  (a)  disposed  on  a  free  surface;  (b)  united  by  an 
intercellular  cement  substance;  (c)  devoid  of  blood-ves- 
sels, though  not  necessarily  of  nerve-terminals. 

Epithelium  of  one  or  another  form  is  normally  present 
on  all  free  surfaces,  save  some  of  those  known  as  synovial ; 
it  invests  the  skin  and  the  mucous  membranes  with  their 
various  diverticula,  lines  the  cavities  of  the  nervous  axis, 
occurs  in  the  organs  of  special  sense,  in  the  cavities  of  the 
th\'roid  and  similar  bodies,  and  (as  endothelium)  lines  the 
serous  surfaces  and  the  cavities  of  the  heart  and  vessels. 
There  are  also  found  in  the  structure  of  various  solid 
organs  (e.  g.,  the  thymus)  masses  of  cells,  which,  while 
they  no  longer  line  free  surfaces,  are  evidently  epithelial  in 
character,  comparative  and  embryological  studies  demon- 
strating their  epithelial  origin.  The  same  may  be  said  of 
the  cells  which  compose  the  greater  portion  of  the  sub- 
stance of  the  liver. 

The  reproductive  elements  have  already  been  mentioned 
as  epithelial  in  origin.  They  are  derived  from  epithelial 
layers  by  processes  diametrically  opposed  in  character; 
those  of  the  male  being  set  free  from  the  layer  in  which 


18  PART    I.      THE    TISSUES. 

they  are  formed,  those  of  the  female  sinking  (for  a  time) 
into  the  subjacent  tissues.  Further  consideration  of  their 
formation  will  be  deferred  until  the  discussion  of  the  organs 
in  which  they  occur. 

The  classification  of  the  epitheha  is  based  on  the  form, 
arrangement,  or  special  modification  of  some  or  all  of  the 
constituent  cells. 
As  regards  the  form  of  the  cells,  epithelia  may  be 

A.  Flattened,  with  flattened  nuclei,  comprising 

1.   Squamous ;  flattened  or  scale-like,  with  bevelled 

edges ;  if  with  vertical  borders,  then  more  properly 

known  as 

,  2.   Pavement;  having  the  form  and  disposition  of 

!'Lvjuir~-<^'p'        tiles.  The  terms  squamous  and  pavement  are  often 

"^ '  ^^T^T^  tivUv'        confounded;  they   are    here    used    to    distinguish 

lUh  Ly-xH--''  clearly  recognizable  differences  of  form. 

B.  Isodiametric,  or  with  nearly  equal  dimensions  in 
"various  directions,  the  nuclei  central  and  spherical  or 
nearly  so.    Known  as 

J-vv  3.  Polyhedral;  also  called  Cuboidal,  Spheroidal, 

"'*  '  and,  from  its  most  frequent  occurrence.  Glandular; 

where  found  on  curved  surfaces  the  cells  are  not  un- 

frequentl}'  wedge-shaped    or    pyramidal  through 

compression, 

C.  Vertically  elongated ;  the  nucleus  undergoing  a  simi- 
lar change  of  form,  and  in  some  cases  situated  nearer  the 
base  than  the  free  end  of  the  cell.    Termed 

4.   Columnar:   the  various  modifications    of  form 
"^  ,^r,  may  be    distinguished    as    cylindrical,   prismatic, 
club-shaped,  etc. 


CHAI'TEK    II.      THE    KI'lTlIELIA.  19 

As  regards  the  arrangement  of  cells,  epithelia  ma}-^  be 

1 .  Simple ;  composed  of  a  single  layer  of  pavement, 
cuboidal,  or  columnar  cells. 

2.  Transitional;  composed  of  a  layer  a  few  cells 
in  depth,  the  constituent  cells  varying  but  slightly 
in  form.  '  ^ 

3.  Stratified:  several  cells  deep,  and  more  or  less 
distinctly  definable  into  layers .  called  Stratified 
Squamous  or  Stratified  Columnar,  according  to 
the  form  of  the  most  superficial  cells. 

There  are  numerous  special  modifications  of  epithelial 
cells  occurring  in  particular  localities,  as  in  the  organs  of 
special  sense :  such  will  be  described  in  their  proper  con- 
nections.   Of  more  general  occurrence  are  the  following: 

A.  Ciliated  epithelium ;  usualh' columnar,  occasionally 
cuboidal,  rarely  flattened ;  the  free  surface  of  the  cell  is  be- 
set with  hair-like  or  lash-like  prolongations  of  its  proto- 
plasm (cilia),  capable  of  vigorous  flexion  in  one  direction. 

B.  Goblet-cells:  Columnar  (rarely  polyhedral)  mucus- 
secreting  cells  in  which  the  undissolved  mucigen  accumu- 
lates in  the  distal  extremity  of  the  cell,  forming  a  viscid 
transparent  mass,  while  the  protoplasm,  together  with 
the  contained  nucleus,  is  crowded  down  to  the  base  of  the 
cell.  The  extremity  of  the  cell  is  finally  forced  off  b}-  the 
escaping  mucigen,  leaving  a  chalice-shaped  structure. 

C.  Prickle-cells:  the  polyhedral  cells  in  the  deeper  por- 
tion of  stratified  squamous  epithelium  have  their  surfaces 
beset  with  fine  immobile  processes  (prickles)  connecting 
adjacent  cells;  the  latter  being  separated  from  each  other 


20  PART  I.      THE  TISSUES. 

by  intercellular  spaces  and  channels.  By  some  these  pro- 
cesses are  regarded  as  continuations  of  the  protoplasm, 
by  others  of  the  modified  surface  of  the  cell.  They  will  be 
discussed  more  fully  in  connection  with  the  skin. 

The  term  Endothelium  is  applied  to  the  pavement  epi- 
thelium lining  the  vascular,  serous  and  to  some  extent  the 
synovial  surfaces.  Embryological  considerations  led  the 
earlier  histologists  to  regard  it  as  quite  distinct  in  origin 
from  other  epithelia;  more  extended  knowledge  has  shown 
this  distinction  to  be  doubtful,  and  some  have  urged 
that  the  use  of  the  term  endothelium  should  be  abandoned 
altogether.  There  are  reasons  why  it  appears  to  be  well 
to  retain  the  term  with  the  significance  above  indicated : 
as  thus  applied,  it  can  also  be  defined  as  a  layer  of  con- 
nective tissue  cells  on  a  free  surface,  a  definition  that  will 
receive  further  consideration  in  another  chapter. 

Serous  endothelium,  when  viewed  from  above,  presents 
a  mosaic  of  polygons  whose  various  dimensions  are  nearly 
equal,  and  whose  boundary  lines  are  straight  and  short. 
Vascular  endothelium  is  made  up  of  cells  elongated  in 
the  direction  of  the  vessel  which  ihev  line,  and  tapering  at 
each  extremity.   The  boundary  lines  are  long  and  sinuous. 

The  terms  epithelioid  and  (more  commonly)  endotheli- 
oid  are  applied  to  layers  of  connective  tissue-corpuscles 
which  resemble  epithelium  in  their  regularity,  but  are  not 
found  upon  a  free  surface.  The  description  of  these  layers 
does  not,  however,  properly  belong  to  the  discussion  of 
the  epithelia. 


CIIAPTKR    II.      THE    KPITIIKLIA.  21 

The  statements  above  made  concerning  the  form  and  ar- 
rangement of  epithelial  cells  are  purposely  quite  general  in 
character:  it  will  rejidilv  be  understood  that  within  the 
groups  indicated  we  may  have  great  variety  of  detail, 
the  epithelia  of  different  localities  having  in  almost  all 
cases  their  own  individual  characteristics,  by  which  they 
can  in  many  instances  be  clearly  identified,  as  will  be 
pointed  out  from  time  to  time  in  the  discussion  of  the  or- 
gans containing  them.  It  should  also  be  clearly  under- 
stood that  the  differences  in  form  described  are  separated 
by  no  hard  and  fast  lines ;  flattened,  isodiametric,  and 
elongated  cells  passing  into  each  other  by  gradations  at 
times  so  slight  that  it  is  often  difficult  if  not  impossible  to 
say  of  a  particular  cell  whether,  for  instance,  it  should  be 
termed  columnar  or  poK-hedral ;  and  the  continuation  of 
the  same  simple  layer  affording  in  some  instances  exam- 
ples of  each  in  turn  of  the  three  fundamental  forms. 

As  regards  the  arrangement  of  the  cells,  epithelia  can 
never  be  said  to  pass  into  each  other  so  gradually,  since  a 
layer  of  cells  must  always  be  either  one  cell  deep  or  more 
than  one  cell  deep.  The  alternative  is  usually  between  a 
simple  epithelium  and  one  composed  of  more  or  less  dis- 
tinct strata,  the  cells  on  the  surface  differing  markedly 
from  those  at  the  base :  an  arrangement  transitional  be- 
tween these  is  usually  defined,  as  stated  on  a  preceding 
page.  Practicalh'  the  term  transitional  is  by  most  histol- 
ogists  applied  only  to  the  epithelium,  peculiar  alike  in  struct- 
ure and  function,  which  lines  the  urinar}'  tract,  in  connec- 
tion with  which  it  will  be  discussed. 


22  Part  i.    the  TrsstJEis. 

The  various  epithelia  of  the  body  are  of  exceedingly  di- 
Terse  embryonic  origin,  as  will  be  pointed  out  in  a  subse- 
quent chapter:  but  however  diverse  their  origin,  and  how- 
ever much  the\'  may  differ  in  the  form  and  arrangement  of 
the  elements  of  the  adult  structure,  they  in  each  case  origin- 
ate as  a  single  la^-er  of  isodiametric  cells  resting  upon  the 
subjacent  membrane  or  its  precursor.  Their  subsequent 
multiplication,  if  by  cleavage  in  a  vertical  plane  only,  gives 
rise  to  a  simple  Xaryer  of  polyhedral,  or,  by  lateral  pressure, 
columnar  cells:  if  cleavages  occur  also  in  a  horizontal 
direction,  or  irregularh',  a  stratified  epithelium  is  the  re- 
sult. 

The  power  of  multiplication  is  probabh'  retained 
throughout  life  b}'  all  epithelia :  by  some,  however,  it  may 
be  manifested  at  times  only  as  a  process  of  repair,  or,  if 
normally  constant,  may  proceed  but  slowly:  b}'  others, 
however,  the  constant  and  more  or  less  rapid  production 
of  new^  cells  is  a  part  of  its  chief  function.  This  is  nota- 
bly the  case  with  the  stratified  squamous  epithelia,  whose 
outermost  cells  are  continually  being  discharged  from  the 
surface,  either  by  exfoliation  or  by  their  union  to  form 
such  solid  masses  as  the  hairs  and  the  nails. 


CHAPTKR    III.      THE   CARTILAGE   GROI'P.  23 


CHAPTER  III. 
THE  CARTILAGE  GROUP. 


The  presence  of  an  Intercellular  cement-substance  has 
been  mentioned  as  characteristic  of  the  structure  of  the 
epithelia  :  this  substance,  is,  however,  ahvavs  small  in 
quantity,  and  mayor  may  not  be  the  product  of  the  activ- 
ity of  the  cells  themselves.  Where  the  epithelial  elements 
give  rise  to  formed  products  of  any  considerable  extent, 
thej'  are  either  deposited  in  a  solid  form  as  a  direct  invest- 
ment of  the  cell  itself  (e.  g.,  the  keratin  layer  forming  the 
wall  of  a  squamous  cell  from  the  outer  surface  of  a  strati- 
fied epithelium),  or  are  discharged  on  the  free  surface  as 
secretions  either  in  a  semi-solid  form  (e.  g.,  the  mucigen  of 
the  goblet  cells),  or  as  a  more  complete  solution  (e.  g.,  the 
products  of  most  glands). 

The  tissues  of  the  group  noAv  to  be  considered,  on  the 
other  hand,  have  it  as  their  chief  if  not  as  their  sole  com- 
mon characteristic  that,  however  much  the\^  may  differ 
in  appearance  and  consistency  (from  transparent,  color- 
less, almost  semi-fluid  gelatinous  tissue  to  hard,  white, 
opaque  dentine),  they  consist  in  every  case  of  cells  or  cor- 
puscles, which,  as  their  chief  activity,  give  rise  to  a  rela- 
tively large  amount  of  formed  products  deposited  as  an 
intercellular  matrix. 


24  Part  i,    the  tissues. 

The  matrix  thus  formed  is  at  first  of  slight  consistency, 
and  homogeneous  in  structure.  It  may  become  strength- 
ened by  subsequent  direct  modification  in  density  and  ten- 
acity; by  calcification,  lamination,  or  fibrillation;  or  by 
various  combinations  of  these  methods.  The  various 
changes  in  the  structure  of  the  matrix  thus  produced,  with 
the  concomitant  changes  in  the  form,  number  and  arrange- 
ment of  the  corpuscles,  give  rise  to  a  variety  of  tissues 
which  have  an  essential  community  of  function  directly 
associated  with  their  fundamental  community  of  struc- 
ture. The  former  is  illustrated  by  the  facts  of  compara- 
tive anatomy,  w^hich  show  that  these  tissues  replace  each 
other  to  a  very  great  extent  in  different  vertebrates ;  and 
the  latter  by  the  substitutions  and  adventitious  growths 
that  occur  abnormally  in  the  human  body.  As  will  readily 
be  supposed,  they  have  a  common  embryonic  origin. 

The  term  Skeletal  Tissues  is  here  applied  to  the  mem- 
bers of  this  group  because  of  their  chief  function.  Thej' 
are  the  framework  tissues  of  the  body,  investing  and  pen- 
etrating every  organ,  and  supporting  and  protecting  every 
other  tissue.  On  account  of  their  continuity,  and  the  part 
they  play  in  binding  the  organs  of  the  body  to  each  other 
they  are  also  widely  known  as  the  Connective  Tissues ; 
a  term,  however,  more  appropriately  used  in  its  original 
significance,  as  applied  to  one  of  the  principal  divisions  of 
the  group. 

As  will  be  readily  inferred  from  what  has  already  been 
stated,  the  classification  of  the  skeletal  tissues  is  based  in 
part  on  differences  in  the  form  and  disposition  of  the  cor- 


CIIAPTKR   III.      CARTILAGE.  25 

puscles,  and  in  part  on  ditit'erences  in  the  structure  of  the 
rnatrix.  It  is  characteristic  of  the  group  as  a  whole  that 
the  corpuscles  tend  to  branch  irregularly  and  to  remain  or 
become  united  b}-  the  prolongation  of  their  branches  into 
more  or  less  extensive  protoplasmic  networks  whose  nodes 
are  the  nucleated  bodies  of  the  corpuscles.  It  is  within 
the  meshes  of  this  network  that  the  matrix,  whether  sim- 
ple or  complex  in  structure,  is  deposited. 

While,  however,  the  branching  and  intercommunication 
of  the  corpuscles  just  mentioned  is  shown  by  the  evidence 
alike  of  comparative  anatomy,  of  embryology,  and  of  path- 
ology' to  be  characteristic  of  each  of  the  skeletal  tissues  in 
some  animals  or  in  some  stages  and  conditions,  we  find  in 
the  healthy  adult  bod}' of  man  and  the  mammals  generallv 
differences  in  this  and  associated  respects  which  divide  the 
tissues  in  question  into  two  principal  groups  quite  sharply 
distinguished  from  each  other  as  regards  the  primary 
structure  of  the  forms  included  in  each,  although  in  some 
cases  the  two  types  are  secondarih^  intermingled. 

In  the  first,  or  Cartilage  Group,  the  corpuscles,  which 
are  always  of  one  kind  only,  (fixed  corpuscles)  are  usually 
either  spheroidal  in  form,  or,  as  the  result  of  pressure, 
polyhedral  or  flattened :  in  either  case,  however,  they  are 
simple  in  outline;  and  are  isolated  or  at  least  discon- 
nected :  in  rare  cases  they  are  sparingU'  branched  and 
connected.  The  matrix  is  firm,  elastic,  primarily  homo- 
geneous and  finely  granular,  apparent^  structureless 
(though  some  facts  indicate  an  internal  structure  not  yet 
clearly  demonstrated):   whether  more  or  less  dense,  it  is 


26  PART    I.      THE    TISSUES. 

alwa^'-s  permeable  b}'  diffusion  to  the  nutrient  plasma  on 
which  the  corpuscles  depend  for  sustena  ice,  Ij'-mph-chan- 
nels  being  absent  (or  very  doubtfully  present),  and  a  regu- 
lar blood-supply  wanting,  although  large  masses  are  some- 
times sparingly  penetrated  by  blood-vessels:  it  is  some- 
times secondarily  reinforced  by  the  intermingling  of  fibrous 
bujodles,  or  the  deposition  of  lime  salts. 

According  to  the  extent  to  which  the  matrix  is   devel- 
oped, and,  to  the  character  of  its  reinforcement,  when  this 
occurs,  the  various  members  of  the  cartilage  groLip  may 
be  classified  as  follows: 
■    A.   Matrix  simple,  or  not  reinforced  b\'  fibres  : 

1.  Cellular  Cartilage:  Matnx  very  scanty,  con- 
sisting only  of  thin  laj-ers  deposited  around  the 
corpuscles,  which  are  numerous  and  relatively 
large:  sometimes  called  parenchymatous  carti- 
lage from  its  resemblance  to  the  parenchyma  of 
plants :  occurs  in  the  embr\^os  of  man  and  many 
vertebrates  and  in  the  auricular  cartilages  of  small 
mammals,  as  well  as  elsewhere  in  the  permanent 
skeleton  of  some  of  the  lower  vertebrates. 

2.  Hyaline  Cartilage:  Matrix  abundant,  though 
varying  in  quantity,  the  corpuscles  solitary'  or 
gathered  into  small  groups  as  the  result  of  recent 
subdivisions :  translucent,  white  or  bluish-white  in 
color,  brittle,  firm  and  elastic :  the  typical  form  of 
cartilage.  Occurs  in  the  encrusting  cartilages  of  all 
freely  movable  joints  (the  corpuscles  in  them  being 
numerous,  small  and  near  the  surface  flattened 
verticalh');   in  the  laryngeal  cartilages,  with  two 


CHAPTFK    III.      CARTII.ACK.  27 

exceptions  to  be  noted  later;  in  the  nasal,  costal, 
tracheal  and  bronchial  cartilages ;  and  in  nearly 
all  toetal  cartilages. 

3.  Calcified  Cartilage:  Hyaline  cartilage  is  fre- 
(luently  reinforced  in  old  age,  both  in  man  and  in 
the  mammals  generally,  by  the  regular  deposition 
in  the  matrix  of  nodules  of  lime  salts.  This  pro- 
cess occurs  regularly  in  some  of  the  lower  verte- 
brates to  such  an  extent  as  to  give  rise  to  a  tissue 
almost  bonelike  in  density  and  forming  the  princi- 
pal framework  of  the  body. 
B.  Matrix  reinforced  by  the  intermingling  with  it  in 
smaller  or  larger  proportions  of  fibrous  bundles: 

4-.  Reticular  Cartilage :  Matrix  continuous,  pene- 
trated irregularly  by  a  network  of  i^ellow, elastic 
fibres;  the  corpuscles  relatively  large  and  near 
together,  approaching  cellular  cartilage  in  this  re- 
spect. Occurs  where  great  flexibility  and  toughness 
combined  with  elasticity  are  called  for ;  inthecartil- 
ages  of  the  external  ear,  in  the  Eustachian  tube,  in 
the  epiglottis  and  in  the  cartilages  of  Wrisberg  and 
of  Santorini  in  the  larjmx.  On  account  of  its  color 
and  structure  this  tissue  is  sometimes  spoken  of  as 
yellow  fibro- cartilage,  and  on  account  of  its  phy- 
sical properties  as  elastic  cartilage. 

5.  Fibro-Cartilage  proper:  Matrix  largeh- replaced 
by  bundles  of  white  fibres ;  the  corpuscles  small 
and  few  in  number,  resembling  those  of  h^'aline 
cartilage  in  appearance.  Occurs  where  great  ten- 
acity combined  with  elasticity  and  moderate  flexi- 


28  PART    I.      THE    TISSUES. 

bilitA''  are  needed  ;  in  the  intervertebral  disks,  in  in- 
terarticular  masses,  at  the  margins  of  ball-and- 
socket  joints,  in  the  sacro-iliac  articulations,  in  the 
sx^mph^'sls  pubis.  As  distinguished  from  the  pre- 
ceding it  is  sometimes  termed  white  fibro-cartil- 
age. 

Fibro-cartilage  may^  also  be  described  as  consist- 
ing of  masses  of  interwoven  bundles  of  fibrous  tis- 
sue with  small  nodules  of  hyaline  cartilage  inter- 
spersed sparingly  in  the  meshes.  So  considered,  it 
may  be  regarded  as  a  mixture  of  fibrous  and  cartil- 
age tissues. 

Cartilage  always  originates  as  a  mass  of  contiguous 
spheroidal  or  polyhedral  cells.  As  development  proceeds 
the  cells  are  seen  to  be  separated  by  thin  layers  of  a  color- 
less substance,  which  is  formed  b}-  the  deposition  about 
each  cell  of  a  la\'er  of  matrix  substance  known  as  the 
capsule  of  the  cell ;  cellular  cartilage  never  proceeds  be- 
3'ond  this  stage:  in  the  case  of  h\'aline  cartilage  the  mat- 
rix substance  accumulates  between  the  capsules  bx'^  exter- 
nal deposition,  or  else  is  formed  b}'  the  gradual  transfor- 
mation and  removal  of  the  capsules.  Reticular  cartilage  is 
always  pre-formed  as  h^^aline  cartilage,  the  elastic  fibres 
afterwards  appearing  in  the  matrix.  In  fibro-cartilage  the 
cartilaginous  substance  and  the  fibrous  tissue  are  said  to 
appear  simultaneousl3\ 

Cartilage  grows  by  cell-division,  which  can  without  diffi- 
cult}^ be  seen  to  have  been  in  progress  during  life  in  any- 
good  section  of  hj^aline  cartilage,    the  corpuscles  being 


CHAPTER    III.      CARTILAGE.  29 

tbund  in  «jroups  of  two,  four  or  more,  so  related  as  to 
clearly  indicate  their  recent  origin  :  in  some  instances  two 
cells  each  with  a  proper  capsule  can  be  found  within  the 
capsule  of  the  cell  from  whose  division  they  were  devired. 
Such  interstitial  growth  doubtless  proceeds  more  rapidly 
in  most  cases  near  the  surface  than  in  the  deeper  portions 
of  the  cartilage :  it  may  suffice  merel  r  for  the  constant  re- 
newal of  the  tissue,  or  may  proceed  with  sufficient  rapid- 
ity to  give  rise  to  actual  increase  in  size.  Growth  in  this 
sense  is  believed  by  some  to  take  place  chiefly  by  apposi- 
tion :  that  is  by  the  deposition  upon  the  surface  of  new 
cartilage  substance. 

It  is  customary  to  mention  in  connection  with  t1ie  de- 
scription of  cartilage  that  the  matrix  consists  chiefly  of  a 
substance  frequently  called  chondrogen,  and  said  to  yield 
chondrin  on  boiling;  the  latter  is  defined  as  a  member  of 
the  gelatin  group  of  compounds.  Gelatin  is  itself  ob- 
tained chiefly  by  boiling  the  fibrous  tissues,  which  are  rich 
in  its  antecedent,  collagen.  By  some  chemists  the  matrix 
of  cartilage  is  regarded  as  also  consisting  largely  of  colla- 
gen, the  so-called  chondrin  being  regarded  as  only  an  im- 
pure or  slightly  modified  gelatin.  The  matter  is  one  that 
has  no  direct  bearing  upon  the  structure  of  tlie  tissues  in 
question  (as  far  as  our  present  knowledge  goes)  but  it  is 
well  for  the  student  to  understand  what  is  meant  by  the 
terms  mentioned. 

A  cartilage,  in  the  anatomical  sense  of  the  word,  is  an 
organ :  that  is  to  say,  a  particular  part  of  the  body  hav- 
ing a  definite  form  and  function.    As  such,  its  description 


30  PART  I.      THE    TISSUES. 

might  with  propriety  be  deferred  to  the  second  part  of  this 
book :  in  the  case  of  this,  however,  as  of  some  other  organs 
consisting  chiefly  (though  not  solely)  of  a  single  tissue,  it 
will  be  for  various  reasons  desirable  to  discuss  its  struct- 
ure in  connection  with  that  of  its  prevalent  tissue. 

A  cartilage,  then,  may  be  defined  as  a  mass  of  cartilage 
tissue  having  a  definite  and  reg'ular  form.  It  is,  especially 
where  composed  of  hyaline,  calcified,  or  reticular  cartilage, 
usually  covered  by  the  perichondrium,  a  thin  fibrous  mem- 
brane moderately  rich  in  blood  vessels,  which  are  the  sole 
or  chief  source  of  nutriment  for  the  mass :  the  elastic  fibres 
of  reticular  cartilage  are  continuous  with  this  membrane. 
A  cartilage  is  always  devoid  of  bloodvessels,  though  large 
cartilages  are  sometimes  excavated  by  spaces  of  greater 
or  less  extent,  through  which  bloodvessels  pass,  accom- 
panied by  lymph-vessels  and  sometimes  by  fat,  forming 
what  is  sometimes  termed  a  "cartilage  marrow."  It  is 
always  devoid  of  nerves  and  insensitive  to  pain. 


CHAI'TKK    IV.      FIHROl'S   TISSUES.  31 


CHAPTER  IV. 
THE  FIBROUS  TISSUES. 


In  the  second  of  the  two  groups  of  skeletal  tissues  above 
indicated,  the  Fibrous  Tissue  Group,  in  addition  to 
the  corpuscles  primarily  associated  with  the  formation  ot 
the  tissue  and  permanently  located  in  it  (hence  called  fixed 
corpuscles),  there  ma\'  be  present,  in  some  members  of  the 
group  at  least,  characteristic  accessor}'  or  adventitious 
corpuscles  of  various  kinds.  The  fixed  corpuscles  are 
always  irregular  in  form,  with  lamellar  or  filamentojus 
branches,  the  latter  frequently  connecting  with  similar 
processes  from  adjacent  cells,  thus  forming  a  more  or  less 
continuous  network.  The  matrix  isalwaj's  homogeneous, 
transparent  and  yielding  in  the  embryonic  state,  but  very 
early  becomes  penetrated  by  fine  fibrillae  running  irregu- 
larly in  various  directions :  the  fibrillation  is  in  most  cases 
extensive,  the  matrix  finally  consisting  chiefly  of  a  mass  of 
fibres  variously  disposed  in  bundles,  in  more  or  less  closely 
felted  layers,  or  in  clearly  defined  laminae.  In  some  cases  the 
fibrillation  of  the  matrix  is  regularly  followed  by  calcifi- 
cation. 

The  number  of  tissues  which  agree  in  having  the  general 
structure  indicated  as  characteristic  of  the  Fibrous  Tissue 
group  is  larger  than   that  of  all  the  other  tissues  of  the 


32  PART  I.     THE  TISSUES. 

body  put  together.  Three  of  them,  namely,  corneal  tis- 
sue, bone  tissue  and  dentine,  resemble  each  other  and  differ 
from  all  the  rest  in  the  fact  that  the  fibres  formed  by  the 
union  of  the  fibrillae  are  always  very  minute  and  are  closely 
felted  together  to  form  definite  lamellae  between  which  or 
exterior  to  which  the  fixed  corpuscles  are  situated :  the 
first  of  these  is  remarkable  for  its  extreme  transparency ; 
the  other  two  are  normally  and  extensively  calcified,  form- 
ing tissues  of  great  density  and  firmness.  Their  further 
description  will  best  be  deferred  to  a  subsequent  chapter. 

The  remaining  members  of  the  group  constitute  the  Fi- 
brous Tissues  proper  or  the  Connective  Tissues  in  the 
more  limited  sense  in  which  the  term  may  best  be  used. 
Of  these  one  is  chiefly  if  not  entirely  embryonic,  existing  in 
the  adult  human  body  only  in  an  extremely  modified  form. 
As  it  is  an  essential  constituent  of  an  important  foetal 
structure,  it  merits  a  description  as  a  distinct  form  of  con- 
nective tissue:  and  since  it  is  the  precursor  of  most  of  the 
others,  its  discussion  may  properly  precede  their  classifica- 
tion and  description. 

Mucous  Tissue  (or,  as  it  is  also  called,  gelatinous  tis- 
sue) :  the  matrix  is  at  first  homogeneous,  transparent  and 
semi-fluid  in  consistency ;  it  is  described  as  albuminous  in 
composition,  with  the  addition  of  mucin:  fibres  very  early 
begin  to  appear  in  it,  their  mode  of  formation  being  not 
yet  fully  determined :  the  fixed  corpuscles  are  irregular, 
branching,  connected  by  their  slender  processes  into  a  net- 
work :  in  addition  there  are  to  be  seen  here  and  there  in 
thejnatrix  scattered  isolated  corpuscles  which  in  the  fresh 


CHAI'TKR    IV.      FIHROUS  TISSUES.  33 

tissue  ma}^  be  seen  to  move  through  the  jelly-Hke  sub- 
stance with  an  irregular  or  amoeboid  motion ;  these  are 
the  migratory  corpuscles  or  leucocytes  which,  as  we 
shall  see,  are  characteristic  of  the  connective  tissues  as  a 
group.  Mucous  tissue  constitutes  an  important  factor  of 
the  umbilical  cord,  where  it  forms,  under  the  name  of  the 
jelly  of  Wharton,  the  largest  portion  of  the  mass  lying  be- 
tween the  epithelium  upon  the  surface  and  the  bloodvessels 
in  the  centre,  up  to  the  fifth  month  ;  later  the  fibrillation' 
which  has  already'  begun  about  the  vessels  and  near  the 
surface  penetrates  the  w^hole  mass  mo^-e  and  more  exten- 
siveh' :  but  a  certain  amount  of  sparingly  fibrillated  mu- 
cous tissue  always  persists.  While  mucous  tissue  is  a  fre- 
quent constituent  of  the  skeletal  framework  of  some  of  the 
lower  animals,  it  is  represented  in  the  adult  human  body 
(save  as  a  constituent  of  morbid  growths)  by  the  following 
structures,  if  at  all. 

The  vitreous  body  (or  so-called  vitreous  humor)  of  the 
eye  may  best  be  regarded  as  a  modified  form  of  mucous 
tissue.  In  the  embr\'o  it  possesses  for  a  time  all  the  char- 
acteristics of  that  structure ;  but  in  the  adult  the  matrix 
undergoes  watery'  degeneration,  and  fibres  are  extremely 
rare;  fixed  corpuscles  are  altogether  wanting,  the  only 
corpuscular  elements  present  being  a  few  leucocytes.  The 
centre  of  the  intervertebral  djscs  of  fibro-cartilage  con- 
tains a  soft  and  yielding  mass  sometimes  regarded  as  a 
form  of  mucous  tissue:  it  lacks,  however,  some  of  the 
essential  features  of  that  structure  and  may  best  be  re- 
garded as  the  remains  of  the  notochord,  the  embryonic 
precursor  of  the  vertebral  column,  which,  while  it  differs 


34  PART    I.      THE    TISSUES. 

in  some  respects  from  cellular  cartilage,  approaches  more 
nearly  to  it  than  to  mucous  tissue  as  here  defined.  The 
pulp  of  the  teeth  also  consists  in  part  of  a  modified  form 
of  gelatinous  tissue,  which  will  be  more  fully  described  in 
connection  with  those  organs. 

Passing  now  to  the  consideration  of  the  fibrous  tissues 
proper,  as  defined  by  the  limitations  recently  indicated,  it 
is  important  to  note  at  the  outset  that  the  fibres  which  in 
every  case  enter  so  largel}^  into  their  composition  are  of 
two  kinds,  both  of  which  are  present  in  most  of  the  tis- 
sues in  question,  though  their  proportions  may  vary  ex- 
ceedingly ;  the  characters  of  the  tissues  included  in  the 
group  being  in  great  measure  based  upon  the  proportion 
of  the  two  kinds  of  fibres,  and  the  modes  of  their  disposi- 
tion. These  are  known  respectively  as  white  fibres  and 
yellow  or  elastic  fibres. 

The  former  are  exceedingly  delicate  unbranching  fila- 
ments of  collagen  (which,  as  has  been  stated,  is  converti- 
ble into  gelatin  by  boiling),  rarely  more  than  a  micron  in 
diameter,  and  often  much  less;  they  are  almost  always 
united  into  bundles  of  varying  size ;  when  so  united  the 
fibres  have  a  silky  appearance,  and  tend  to  assume  a  char- 
acteristic waviness  in  which  all  the  fibres  of  the  bundle  par- 
ticipate in  such  manner  as  to  retain  their  almost  strictly 
parallel  arrangement :  the  bundles  are  again  frequently 
united  together  in  larger  aggregates  sometimes  termed 
trabeculae:  white  fibres  are  very  tenacious  and  entireh' 
devoid  of  elasticity. 

The  elastic  fibres  are  coarser  than  the  white  and  more 
variable  in  thickness,  being  from  one  to  six  micra  in  diam- 


CHAPTER   IV.      FIBROUS  TISSUES.  35 

eter  in  man,  and  in  some  animals  as  much  as  fifteen  micra: 
thev  are  pxismatic  in  form,  and  under  some  circumstances 
appear  to  be  tninsverselv  striated :  they  branch  occasion- 
allv,  and  not  unfrequently  anastomose:  when  not  on  the 
stretch  they  tend  to  Jissume  large  sweeping  curves,  and 
the  free  ends,  which  break  square  across,  curl  up  in  a  ehar- 
a£|er|stic  manner :  like  the  white  fibres,  they  are  often  as- 
sociated in  bundles.  They  are,  as  their  name  implies,  emi- 
nently elastic,  but  are  of  only  moderate  tenacity,  a  bundle 
of  them  being  far  more  easily  broken  across  than  a  bundle 
of  \yhite  fibres  of  the  same  size.  They  are  composed  of  a 
substance  known  as  elastin,  a  complex  nitrogenous  com- 
pound which  is  not  converted  into  gelatin  on  boiling.  The 
elastic  fibres  are  not  readily  affected  by  weak  acids,  as  are 
the  white  fibres. 

But  little  is  as  yet  known  of  the  mode  of  formation  of 
either  kind  cf  fibres.  It  is  b\^  some  held  that  they  are  in 
all  cases  formed  by  the  transformation  of  a  portion  of  the 
protoplasm  of  embryonic  cells,  the  remainders  of  which, 
either  with  or  without  subsequent  increase  in  size,  become 
the  fixed  corjjuscles  of  the  tissue  in  which  they  are  found  ; 
this  view,  however,  is  urged  not  so  much  on  account  of 
observations  directly  supporting  it  as  of  the  conviction  on 
the  part  of  its  most  positive  adherents  that  the  entire  liv- 
ing body  consists  and  must  consist  onh^  of  protoplasm 
and  the  immediate  products  of  protoplasmic  changes: 
there  are,  moreover,  some  facts  very  difficult  of  explana- 
tion from  this  standpoint.  On  the  other  hand,  it  is  claimed 
that  fibrillation  ma}'  and  does  result  from  a  chemical 
and  physical  change  in  a  part  of  the  homogeneous  ground- 


36  PART  L     THE  TISSUES, 

substance  of  the  matrix  along  lines  not  in  actual  contact 
with  any  mass  of  protoplasm ;  although  it  is  freely  ad- 
mitted that  the  proximity  of  such  masses  (the  corpuscles) 
may  have  a  decided  influence  in  initiating  and  determin- 
ing such  a  process.  In  the  case  of  the  elastic  fibres,  there 
is  good  reason  for  the  view  (first  proposed  by  Ranvier) 
that  the  elastin  is  deposited  in  the  matrix  in  the  form  of 
small  globules,  which  later  fuse  together  to  form  fibres : 
this,  if  true,  will  account  for  the  transverse  striation 
already  mentioned. 

The  corpuscles  of  the  fibrous  tissues  also  demand  pre- 
liminary consideration.  It  has  already  been  indicated 
that  each  tissue  is  characterized  by  the  presence  of  a  spe- 
cific form  of  fixed  corpuscle  peculiar  to  it ;  in  the  case  of 
mucous  tissue  the  presence  of  migratory  corpuscles,  or,  as 
they  are  very  frequently  termed,  leucocytes  has  been  men- 
tioned as  a  feature  in  which  this  tissue  may  be  taken  as 
a  type  of  the  group :  and  reference  has  been  made  to  other 
accessory  or  adventitious  corpuscles.  The  fixed  corpus- 
cles proper  to  each  tissue  may  best  be  described  in  connec- 
tion with  its  definition:  the  nature  and  origin  of  leuco- 
cytes will  be  considered  later  in  connection  with  the  de- 
scription of  the  lymph  and  the  blood :  the  accessory  cor- 
puscles now  call  for  discussion.  They  are  often  spoken  of 
as  modified  fixed  corpuscles :  but  while  one  form  is  cer- 
tainly and  others  are  possibly  derived  from  these  bodies, 
there  is  reason  to  question  whether  those  of  one  (if  not  of 
more  than  one)  kind  are  not  modified  leucocytes:  omit- 
ting further  discussion  of  their  origin  (save  in  one  instance) 
they  may  be  described  as  follows. 


CHAPTER  IV.      FIBROUS  TISSUES.  37 

What  are  known  as  fat  cells  arc  fornied  by  the  fatty 
transformation  of  the  protoplasm  of  certain  of  the  fixed 
corpuscles  of  one  or  more  of  the  fibrous  tissues:  small 
droplets  of  oil  at  first  appear  scattered  in  the  cell-body; 
these  become  more  numerous  or  larger,  finally  fusing  in 
one  large  mass,  the  nucleus  being  crowded  to  one  side,  and 
'the  residual  protoplasm  forming  a  thin  pellicle  or  cell-wall. 

Under  the  name  of  plasma  cells  arc  included  certain 
corpuscles  having  elongated  and  sometimes  slightly 
branching  bodies  with  central  oval  nuclei,  the  protoplasm 
of  which  contains  a  large  number  of  small  vacuoles  (of 
varying  size)  which  contain  a  clear  fluid  probabh'  similar 
in  composition  to  the  lymph  or  blood-plasma,  whence  the 
name  of  these  corpuscles. 

Plasma  cells  were  first  described  by  Waldeyer.  He  in- 
cluded under  that  term  not  only  vacuolated  cells,  but  also 
what  are  now  known  as  granule  cells:  these  are  usually 
spheroidal  in  form  and  devoid  of  branches ;  their  proto- 
plasm is  highly  granular :  on  account  of  the  marked 
afiinity  of  the  granules  for  eosin  (as  well  as  for  many  ani- 
line dyes)  they  are  sometimes  termed  eosinophile  cells;  the 
same  term  has  been  otherwise  applied  in  connection  with 
the  blood,  as  will  be  indicated  later. 

The  name  of  pigment  cells  has  been  given  to  connective 
tissue  corpuscles  (and  to  some  epithelial  cells  as  well) 
characterized  by  the  presence  in  their  protoplasm  of  numer- 
ous rounded  brown  or  black  granules  of  a  substance 
termed  melanin.  Pigment  cells  are  of  very  irregular  form, 
commonl}'  branched,  and  often  exhibiting  amoeboid  mo- 
tion when  examined  in  a  living  condition :  the  pigment 


38  PART    I.      THE    TISSUES, 

granules  are  often  so  numerous  and  so  closely  packed  that 
the  nucleus  and  other  structural  features  are  entirely  hid- 
den; in  some  cases,  however,  they  are  much  less  abundant. 
The  following  classification  of  the  fibrous  tissues  is  based 
upon  the  general  disposition  of  the  fibres  present,  their 
abundance,  the  proportionate  amount  of  the  two  kinds  of 
fibres,  the  details  of  their  distribution,  and  on  the  kinds 
and  relative  quantities  of  the  associated  corpuscles. 

A.  Fibres  varying  in  abundance,  solitary  or  aggregated 
into  bundles  and  trabeculae  running  irregularly  in  various 
directions,  loosely  interwoven  or  more  or  less  closely  felted 
together. 

1.  Areolar  Tissue:  called  by  the  older  histologists 
cellular  tissue:  found  beneath  the  fibrous  layer 
of  the  skin  as  subcutaneous,  beneath  the  mucous 
membranes  as  submucous,  and  the  serous  mem- 
branes as  subserous,  in  the  interspaces  between 
adjacent  organs  as  intermediate,  upon  their  sur- 
faces as  investing  and  forming  their  internal  frame- 
work as  penetrating  areolar  tissue,  it  is  well  nigh 
continuous  throughout  the  entire  body  and  merits 
in  the  strictest  sense  the  name  of  connective  tis- 
sue. The  matrix  consists  in  part  of  a  semi-solid 
homogeneous  ground-substance,  penetrated  in 
every  direction  by  interlacing  bundles  and  trabecu- 
lae of  white  fibres  and  by  elastic  fibres  either  soli- 
tary or  in  bundles :  the  bundles  of  fibres  may  vary 
greatly  both  in  their  total  amount  and  in  the  pro- 
portion of  the  two  varieties ;  but  they  are  never 
so  numerous  but  that  irregular  spaces  of  varying 


CHAPTER  IV.      FinROrS  TISSUES,  39 

A.     Fibres  varying  in  (juantity,  loosely  interwoven,  or 
felted  ( continued) . 

size  termed  areolae  occur  in  so  great  numbers  as 
to  be  practically  continuous ;  a  fact  of  great  im- 
portance in  the  history  of  dropsical  and  other  effu- 
sions. The  whole  structure  is  penetrated  by  blood 
vessels,  lymphatics,  and  by  nerves  passing  through 
it :  it  is  colorless  or  whitish,  filmy  in  texture  and 
of  but  slight  tenacity. 

The  fixed  corpuscles,  or  areolar  tissue  corpuscles 
proper,  are  quite  numerous :  they  are  frequently 
flattened  upon  the  surface  of  bundles  of  fibres  or. 
situated  in  the  angles  where  two  or  more  bundles 
come  together,  in  either  case  the  processes  extend- 
ing along  the  bundles  and  their  subdivisions :  leu- 
cocytes are  of  frequent  occurrence :  plasma  cells 
and  granule  cells  less  so,  except  in  particular  local- 
ities. Pigment  cells  are  not  common  in  areolar 
tissue  proper  in  the  human  body  except  in  certain 
places:  fat-cells  are  very  common  here  and  there  in 
the  areolar  tissue  of  well  nourished  individuals: 
where  they  accumulate  in  particular  localities  they 
give  rise  to 

2.  Adipose  Tissue:  this,  which  may  under  favorable 
conditions  be  formed  wherever  areolar  tissue 
occurs,  but  which  is  most  common  in  connection 
with  the  subcutaneous,  subserous,  and  interme- 
diate regions,  is  in  effect  little  else  than  areolar  tis- 
sue in  which  the  fixed  corpuscles  in  particular  lo- 
calities become  greatly  increased  in  number,  filling 


40  PART  I.      THE    TISSUES. 

A.  Fibres  var\nng  in  quantity,  loosely  interwoven   or 
felted  {continued). 

up  the  areolae,  and  undergo  fatty  transformation, 
giving  rise  to  small  fat  lobules  which  are  gathered 
together  to  form  the  fat  masses  visible  to  the  naked 
eye:  the  blood  supply  of  these  localities  is  always 
greatly  increased,  each  lobule  having  a  capillary 
system  of  its  own,  while  the  fibres  between  the  cor- 
puscles undergo  no  corresponding  increase  in  num- 
ber, and  in  some  cases  are  exceedingly  scanty. 

3.  Retiform  Tissue,  or,  as  it  is  also  called,  reticu- 
lar tissue:  this,  as  its  name  implies,  consists 
chiefly  of  a  network  of  fibres,  or  rather  of  fibre  bun- 
dles and  trabeculae  composed  chiefly  of  what  are 
in  all  probability  most  nearly  allied  to  white  fibres  ; 
true  elastic  fibres  are  very  sparingly  present  or  are 
wanting  altogether,  as  is  the  homogeneous  ground 
substance  characteristic  of  areolar  tissue.  The 
fixed  corpuscles  are  flattened,  adhering  closely  to 
the  surfaces  of  the  bundles  and  trabeculae,  and  are 
often  so  numerous  as  to  form  an  endothelioid  in- 
vestment. Retiform  tissue  may  perhaps  be  re- 
garded as  a  modification  in  the  direction  of  greater 
stability  of  areolar  tissue  (with  which  it  is  often  di- 
rectly continuous),  and  forms  the  internal  frame- 
work of  some  organs,  as  well  as  the  basis  of  the 
two  compound  tissues  known  respectively  as  ade- 
noid tissue  and  marrow.  The  former  of  these  will 
be  described  in  connection  with  the  lymphatic  sa's- 
tem,  and  the  latter  in  connection  with  bone. 


CHAPTER    IV.      FIMKorS   TISSUES.  41 

A.  Fibres   varying  in   (juantity,  loosely  interwoven,  or 
felted  {continued). 

4.  Fibrous  Membrane:  this  differs  from  areolar  tis- 
sue chiefly  in  the  fact  that  the  bundles  and  trabecu- 
lae  of  fibres,  both  white  and  elastic,  are  far  more 
numerous  and  closely  felted  together,  obliterating 
the  areolae  and  leaving  small  space  for  the  inter- 
fascicular ground  substance :  the  fixed  corpuscles 
are  quite  numerous,  but  are,  as  a  rule,  smaller  than 
those  of  areolar  tissue,  and,  with  their  nuclei,  gen- 
erally flattened  in  the  direction  of  the  membrane 
in  which  they  lie ;  both  plasma  and  granule  cells 
may  be  occasionally  present,  and  in  some  cases 
pigment  cells  occur  in  great  numbers  :  fat  cells  but 
rarely. 

The  principal  membranous  tracts  of  the  body 
{e.  g.,  the  mucous  membranes)  which  support  epi- 
thelial or  endothelial  layers  are  sometimes  more  or 
less  clearly  divisible  into  a  stroma,  which  makes  up 
by  far  the  greatest  part  of  the  layer,  and  a  delicate 
film  situated  just  beneath  the  epithelium  and 
known  as  the  basement  membrane,  ormembrana 
propria ;  this  is  sometimes  an  endothelioid  layer  o  £ 
corpuscles,  and  in  other  instances  a  special  conden- 
sation of  the  fibres,  either  white  or  elastic.  Such 
differentiation  does  not  occur  in  those  investing 
membranes  such  as  the  perichondrium  alread\'  re- 
ferred to  in  connection  with  cartilages,  the  similar 
periosteum  of  bones,  etc.,  which  are  not  associated 
with  a  free  surface.    Elastic  membranes  consist 


42  PART    I.     THE    TISSUES. 

A.  Fibres  varying  in   quantity,  loosely  interwoven,  or 
felted  (continued). 

chiefly  or  entirely  of  elastic  fibres,  or,  in  some  cases, 
of  continuous  layers  of  elastin. 
5.  Fenestrated  membrane:  this  form  of  membrane 
is  produced  where,  in  an  otherwise  normally  formed 
fibrous  membrane,  there  are  at  iatervals  of  greater 
or  less  extent  neither  fibre  bundles  nor  ground-sub- 
stance, thus  leaving  rounded  openings  of  various 
size  and  frequency,  as  in  the  omentum  of  man  and 
of  numerous  mammals.  Fenestrated  membranes 
may  also  consist  chiefly,  if  not  entirely,  of  elastic 
tissue,  in  which  the  fibrous  structure  at  times  dis- 
appears in  great  measure,  as  in  the  fenestrated 
elastic  membranes  of  the  blood  vessels. 

B.  Fibres  exceedingly  abundant,  one  or  the  other  kind 
predominating,  aggregated  into  smaller  and  larger  bun- 
dles, which  are  in  a  generally  way  disposed  parallel  wise : 
ground-substance  very  scanty.  Fixed  corpuscles  few,  flat- 
tened between  the  bundles  of  fibres. 

6.  Tendon  tissue  or  white  fibrous  tissue :  as  areo- 
lar tissue  merits  in  the  strictest  sense  the  name  of 
connective  tissue,  so  this,  above  all  others,  merits 
that  of  fibrous  tissue.  It  consists  almost  exclu- 
sively of  white  fibres :  these  are  united  together 
in  small  bundles  by  a  small  quantity  of  ground- 
substance,  each  being  covered  by  an  endothelioid 
layer  of  flattened  corpuscles :  the  smaller  bundles 
are  gathered  together  with  occasional  anastomo- 


CHAPTKK    IV.       FIBROUS   TISSUES.  4.'i 

R.  F'ibres  abundant,  chiefly  of  one  kind,  in  parallel  bun- 
dles {continued). 

ses  into  larger  ones  of  varying  size,  which  are  sep- 
arated by  interstitial  areolar  tissue  continuous 
with  the  investing  sheath  of  the  whole  mass:  elas- 
tic fibres  are  very  sparingly  present,  chiefly  in  the 
penetrating  areolar  tissue.  The  characteristic  fixed 
corpuscles  or  tendon  cells  are  generally  found  in 
rows  whiclj  occupy  the  spaces  between  two  or 
three  contiguous  bundles,  their  branches  taking 
the  form  of  interfascicular  lamellae :  leucocytes  are 
rarely  present,  and  other  forms  of  corpuscles  are 
wanting.  Blood  vessels  and  lymphatics,  and,  in 
some  cases  at  least,  nerve  fibres  follow  the  intersti- 
tial areolar  tissue.  White  fibrous  tissue  is  found 
in  tendons  and  ligaments  and  also  in  tendinous 
aponeuroses  and  fasciae,  which  form  a  transition 
to  fascicular  investing  membranes. 

7.  Elastic  tissue :  this,  as  its  name  implies,  is  com- 
posed chiefly  of  elastic  or  yellow  fibres,  which  are 
arranged  in  bundles  of  varying  size  and  complex- 
ity, the  interstitial  connective  tissue  penetrating 
not  only  the  larger  but  also  the  smaller  bundles, 
and  in  some  cases  separating  individual  fibres: 
small  bundles  of  white  fibres  run  in  the  connective 
tissue  in  various  directions,  and  in  some  cases  at 
least  there  are  well  defined  bundles  arranged  in 
groups  as  in  tendon  tissue  and  running  throughout 
the  whole  length  of  the  structure.  Elastic  tissue 
is  generally  regarded  as  differing  from   all  other 


44  PART  I.     THE  TISSUES. 

B,  Fibres  abundant,  chiefly  of  one  kind,  in  parallel  bun- 
dles {continued). 

forms  of  skeletal  tissue  in  having  no  characteristic 
fixed  corpuscles,  none  being  found  in  it  that  can 
with  certainty  be  regarded  as  such  :  some  authors 
have  described  in  this  connection  flattened  corpus- 
cles found  scattered  in  the  ground-substance  be- 
tween the  fibres  and  in  close  apposition  with  the 
latter;  it  is  not,  however,  certain  that  these  do 
not  more  properly  belong  with  the  interstitial  con- 
nective tissue.  The  term  elastic  tissue  is  by  some 
histologists  applied  alike  to  the  chief  constituent 
of  elastic  ligaments  (in  which  sense  it  is  here  de- 
fined), and  to  that  of  elastic  membranes. 

We  may  now  with  profit  revert  to  the  definition  of  endo- 
thelium given  in  a  previous  chapter,  which  described  it  as 
a  layer  of  connective  tissue  corpuscles  on  a  free  surface. 
We  have  seen  that  the  association  of  corpuscles  with 
fibrous  structures  is  characteristic  of  the  connective  tis- 
sues :  embryological  evidence  shows  that  the  serous  and 
vascular  cavities  are  alike  formed  by  cleavages  or  excava- 
tions of  the  mass  of  embryonic  cells  from  which  the  skele- 
tal tissues  are  derived :  the  tendency  of  connective-tissue 
corpuscles  to  arrange  themselves  in  layers  (properly 
termed  endothelioid)  has  been  mentioned  in  connection 
with  such  structures  as  basement  membranes,  the  investing 
layers  of  small  tendons  and  tendon-bundles,  etc. :  and,  as 
we  shall  see  later,  the  direct  continuity  between  endothe- 
lioid cells  and  connective  tissue  corpuscles  can  be  observed 


CHAPTER    IV.      FIUKOUS   TISSUES.  45 

at  the  free  extremities  of  lymphatics  and  at  the  margins 
of  synovial  surfaces:  it  can,  therefore,  be  readily  under- 
stood how  connective  tissue  corpuscles  should  form  a  con- 
tinuous layer  under  such  exceptionally  favorable  condi- 
tions as  those  found  on  the  free  surface  of  a  membrane. 
Some  characteristic  activities  of  the  corpuscles  found  upon 
serous  surfaces  will  be  described  in  connection  with  the 
lymph  and  the  blood. 

We  know  at  present  but  little  of  the  duration  of  an^'  of 
the  fibrous  tissues,  or,  indeed,  of  the  skeletal  tissues  in 
general.  Consisting  largely  of  the  constituents  of  the 
matrix,  which  some  histologists  regard  as  formed  and  in 
a  certain  sense  not-living  substances,  it  has  been  held  that, 
once  established,  they  may  endure  as  long  as  the  body 
lasts.  It  is  possible  that  this  is  the  case,  to  some  extent 
at  least,  and  that  the  tendon  fibres,  for  example,  of  our 
old  age  are  the  identical  tendon  fibres  of  our  childhood : 
on  the  other  hand,  the  tendons  of  the  child  certainly  in- 
crease both  in  thickness  and  in  length  toward  manhood; 
and  the  mechanism  by  which  interstitial  increase  takes 
place  is  certainly  adequate  for  interstitial  replacement  as 
well ;  we  have,  however,  at  present  no  certain  evidence  of 
any  such  mechanism  of  absorption  or  removal  of  worn- 
out  fibres  (if  such  there  be)  as  a  method  of  replacement 
would  imply.  Whether  the  matrix  of  a  fibrous  tissue  is  to 
be  reg£irded  as  living  or  not  depends  entirely  on  what  we 
mean  by  a  word  for  which  no  generally  accepted  defini- 
tion has  3'et  been  given  :  it  is  certain,  however,  that  under 
certain  conditions   of  defective  nutrition  such  structures 


4!6  PART    I.     THE    TISSUES. 

undergo  marked  changes  to  which  the  name  of  death  is 
certainly  not  inappropriate. 

These  changes  are  generally  regarded  as  due  primarily 
to  the  death  of  the  corpuscles :  but  this  merely  shifts  the 
problem.  It  is  difficult  to  conceive  that  any  single  nucle- 
ated mass  of  protoplasm  should  retain  its  powers  una- 
bated for  half  a  century  or  more ;  but  if  it  may,  what  are 
its  probable  activities  in  the  case,  for  example,  of  a  tendon 
cell?  If,  on  the  contrary  (as  seems  more  probable),  it  is 
constantly  renewed,  what  are  the  activities  of  the  succes- 
sive generations  ?  The  phenomena  of  tissue-repair  in  the 
case  of  injuries  throw  some  light  on  the  problem,  but  it  is, 
on  the  whole,  at  present  unsolved.  It  is  desirable,  how- 
ever for  the  student  to  know^  of  its  existence. 

Before  dismissing  the  discussion  of  the  fibrous  tissues  it 
is  proper  briefly  to  mention  a  form  of  tissue  at  first  sup- 
posed to  be  a  member  of  this  group,  and  described  as  a 
modification  of  retiform  tissue:  subsequent  investigations 
have  shown,  however,  that  it  differs  in  important  re- 
spects from  any  of  the  true  connective  tissues  in  structure, 
and  is  of  widely  differing  embryonic  origin.  It  is  the  sus- 
tentacular  tissue  of  the  brain  and  spinal  cord,  and  is 
known  at  present  by  the  name  of  neuroglia.  It  is  com- 
posed entirely  of  branching  corpuscles  and  their  fibrillar 
processes,  known  as  glia-cells:  their  full  description  can 
best  be  given  in  connection  with  the  nervous  tissues  with 
which  they  are  associated. 


CHAPTER    V.      I.AMKLLATKl)   TISSUES.  47 


CHAPTER  V. 

THE  LAMELLATED  TISSUES. 


As  was  stated  in  the  preceding  chapter,  there  are  three 
members  of  the  skeletal  tissue  group  (namely,  corneal  tis- 
sue, bone  tissue  and  dentine),  which,  like  most  of  the 
fibrous  tissues  described  in  that  chapter,  are  character- 
ized bv  extensive  fibrillation  of  the  matrix,  the  fibrillae 
being  even  more  closely  intermingled  than  are  those  of 
ordinary  membranes:  they  differ  from  these  structures, 
however,  as  already  indicated,  in  the  facts  that  the  fibril- 
lae, which  are  always  exceedingK^  fine  and  resemble  most 
nearly  those  composing  white  fibres,  are  never  aggregated 
into  the  bundles  and  trabeculae  which  are  interwoven  to 
form  membranes,  but  are  felted  together  to  form  d_ense 
layers,  having  in  each  case  the  characteristic  fixed  corpus- 
cles situated  between  them  or  exterior  to  them. 

These  lamellar  structures  have  evidently  much  in  com- 
mon with  and  are  originally  derived  from  the  modification 
of  membranes,  as  the  facts  of  comparative  anatomy  and 
embryology  plainly  demonstrate  (two  of  them  being 
clearly  dermal  in  origin  in  man  and  the  lower  animals 
alike) :  the  characters  above  stated,  are,  nevertheless,  of 
such  importance  as  to  warrant  their  separate  considera- 
tion.   The  name  at  the  head  of  this  chapter  is,  therefore, 


48  PART  I.      THE  TISSUES. 

proposed  for  the  group:  but  it  should  always  be  borne 
clearly  in  mind  that  they  are  more  nearly  related  to  the 
fibrous  tissue  group  than  are  either  of  them  to  the  car- 
tilages :  this  is  the  more  important  because  of  the  fact 
that  one  of  them,  osseous  tissue,  largely  replaces  cartilage 
in  the  formation  of  most  of  the  bones  of  the  body,  thus 
giving  rise  to  the  impression  that  these  two  tissues  are 
closely  allied,  and  that  the  latter  is  in  some  way  trans- 
formable into  the  former,  an  error  which  leads  to  much 
unnecessary  confusion. 

^^^  The  formation  of  dentine,  the  most  peculiar  member  of 

j^^^^^^^^jj^he  group,  is  so  intimately  related  to  that  of  the  other  tis- 
io^^JaJm;:^'  gygg  q£  ^YiQ  teeth,  (of  which  it  forms  the  largest  part),  and 
especially  with  that  of  the  pulp  contained  in  the  tooth- 
cavity  that  it  cannot  well  be  discussed  apart  from  these 
associated  structures:  they  will  be  considered  together 
when  the  teeth  are  described  in  connection  with  the  other 
organs  of  the  region  in  which  they  occur. 

g=rr  Since  corneal  tissue  is  found  only  in  the  structure  from 

^r  which  it  derives  its  name,  its  consideration  might  in  like 

Ai,^^^^    manner  with  propriety  be  deferred  until  the  description  of 

"^  the  eye.     There  are,  however,  points  of  resemblance  be- 

I  tween  it  and  bone  tissue  which  make  its  stud}'  desirable 

as  a  preliminary  to  that  of  the  latter,  particularly  as  the 

absence  of  calcification  renders  far  easier  the  recognition 

of  important  details. 

The  transparent  lamellae  which  form  almost  its  entire 
bulk  are  quite  uniform  in  thickness  throughout  the  entire 
cornea :  they  are  composed  of  white  fibres  running  parallel 


CllAPTEK    V.      IwVMKLLATKI)  TISSUES.  49 

to  each  other  in  each  himella,  scj  disposed  that  those  of 
()ne  hunella  cross  those  of  the  nex^t  at  rij^ht  angles,  or 
nearly  so,  in  the  centre  of  the  cornea  ;  toward  the  margins 
thev  cross  at  varying  degrees  of  oblitiuity :  the  lamellae 
near  the  outer  surface  of  the  cornea  are  traversed  oblit|uely 
by  occasional  bundles  of  fibres,  which  thus  unite  them 
together.  Adjacent  lamellae  are  separated  here  and  there 
by  shallow  lens-shaped  spaces  which  occur  frequently,  but 
at  irregular  intervals;  these  may  be  designated  as  lacunae: 
those  lying  between  the  same  two  lamellae  are  connected 
with  each  other  by  numerous  branching  channels,  which 
may  with  equal  propriety  be  termed  canaliculi:  there  is 
thus  formed  a  continuous  system  of  canals  and  spaces 
across  the  entire  cornea  between  each  two  lamellae,  the 
lacunae  of  one  such  system  having  no  definite  relation  in 
position  or  otherwise  to  those  of  the  next. 

In  the  lacunae  lie  the  corneal  corpuscles,  apparently  ad- 
hering to  the  surface  of  one  or  the  other  of  the  two  adja- 
cent lamellae.  They  are  flattened,  with  flattened  nuclei, 
and  irregular  in  outline;  they  branch  freely,  the  branches 
extending  into  the  canaliculi  and  in  many  cases  connecting 
with  those  from  adjacent  corpuscles,  thus  forming  a  pro- 
toplasmic network,  which  is  possibl}'  coextensive  with 
the  canal-system,  but  does  not  completely  fill  it.  The  re- 
mainder of  the  space  forms  a  means  of  distribution  of 
plasma  from  the  blood  vessels  at  the  margin  of  the  cor- 
nea; leucocytes  also  wander  through  the  larger  canals 
from  lacuna  to  lacuna.  Neither  blood  vessels  nor  lymph- 
atics penetrate  the  substance  of  the  cornea,  nor  are  there 
transverse  canals  which  bring  into  communication   the 


50  PART  I.      THE  TISSUES. 

canal-systems  separated  by  the  lamellae.  The  surfaces  of 
the  cornea,  both  anterior  and  posterior,  are  invested  by 
membranes  in  direct  contact  with  the  most  superficial 
lamellae;  but  there  is  no  genetic  relationship  between  the 
membranes  and  the  lamellae  beneath.  Farther  description 
of  these  membranes,  and  of  the  epithelia  supported  by 
them,  w^ill  be  deferred  to  a  subsequent  portion  of  this 
work. 

It  should  be  stated  before  leaving  the  description  of  cor- 
neal tissue  that  the  use  of  the  terms  lacunae  and  canaliculi, 
as  here  applied  to  the  corneal  spaces  and  lymph  channels, 
is  unusual.  It  is  warranted  by  the  resemblance  between 
these  spaces  and  channels  and  those  occurring  in  osseous 
tissue  to  which  these  names  are  commonly  applied ;  and 
is  offered  here  from  the  conviction  that  a  clear  idea  of  the 
resemblances  (and  also  of  the  differences)  of  the  two  tis- 
sues will  aid  materially  in  a  clearer  understanding  of  the 
structure  of  the  latter. 

The  terms  bone  tissue  and  OSSeous  tissue  are  applied 
indifferently  to  the  chief  constituent  of  the  organs  well 
known  as  bones :  like  all  skeletal  tissues,  it  can  be  defined 
by  the  structure  of  the  matrix  and  the  tornis  and  relations 
of  the  fixed  corpuscles. 
„,jjJ^!^iiir*A  The  matrix  consists  of  t^irn  lamejjae  composed  in  part 
of  fijires  (the  mode  of  whose  arrangement  is  not  clearly 
demonstrable),  and  in  part  of  a  hjomogeneous  ground- 
substance  which  is  strongl}'  impregnated  with  lime  and 
other  salts,  calcium  phosphate  being  the  chief  Prolonged 
boiling  converts  the  fibres  into  a  substance  which  has  been 


UJU^I'Aj) 


CHAPTER    Y.      LAMELLATKD   TISSUES.  51 

called  ossein,  but  which  is  probably  an  iinjjurc  form  of 
j^clatin :  the  fibres  are  therefore  allied  to  if  not  identical 
with  white  fi lyres. 

Between  adjacent  lamellae  are  found  frequent  lenticular 
spaces  of  exceedingly  irregular  outline,  the  lacunae  (  for- 
merly erroneously  termed  the  bone-cells);  like  the  similar 
spaces  in  corneal  tissue,  these  are  connected  together  be- 
tween the  lamellae  by  branching  channels,  or  canaliculi: 
in  bone,  however,  such  canaliculi  are  not  only  present  be- 
tween the  lamellae,  but  also  penetrate  them  in  great  num- 
bers, thus  bringing  into  free  communication  lacunae  of 
diflerent  systems;  the  transverse  canaliculi  in  some  cases 
traversing  two  or  more  lamellae  before  their  termination. 
The  transverse  canaliculi  are  both  larger  and  more  numer- 
ous than  the  interlamellar,  in  accordance,  as  will  be  seen, 
with  the  mode  of  nutrition  characteristic  of  osseous  tissue. 

Within  the  lacunae  are  found  the  characteristic  bone 
corpuscles:  these,  like  the  cavities  which  contain  them, 
are  tlattened  and  irregular  in  outline,  with  flattened  nuclei; 
they  can  be  seen  to  branch  in  some  cases,  but  this  occurs 
far  less  freely  than  in  the  case  of  the  corneal  corpuscles ; 
nor  is  there  good  evidenceof  the  connection  of  the  branches 
through  the  canaliculi  to  any  considerable  extent,  as  has 
been  supposed.  Leucocytes  do  not  traverse  the  Ivmph 
channels  of  bone,  nor  are  any  other  corpuscles  present. 

The  lamellated  and  calcified  fibrous  matrix,  with  its 
characteristic  lacunae  and  canaliculi,  and  the  enclosed  cor- 
puscles, are  the  essential  structural  factors  of  osseous  tis- 
sue wherever  found  :  and  in  some  of  the  lower  vertebrates 


52  PART    I.      THE    TISSUES. 

bones  occur  which  consist  simph-  of  a  few  parallel  lamel- 
lae. In  man  and  the  higher  animals,  how^ever,  these  fac- 
tors are  always  arranged  in  one  of  two  methods  which 
are  so  constant  as  to  constitute  two  distinct  forms  of 
osseous  tissue  called  respectiveh'  dense  and  spongy  bone. 

In  the  former  the  great  majority  of  the  lamellae  are  ar- 
ranged concentrically  to  narrow  tubular  spaces  known  as 
Haversian  canals;  these  contain  blood  vessels,  l^^mphatics 
and  a  small  quantity  of  another  tissue  presently  to  be  de- 
scribed ;  they  are  of  varying  length,  running  in  a  general 
way  parallel  to  the  surface  of  the  mass  in  which  they  oc- 
cur, and  anastomosing  frequently :  Each  is  surrounded  by 
several  lamellae  whose  lacunae  are  put  into  communica- 
tion with  it  by  means  of  transverse  canaliculi.  A  Haver- 
sian canal  and  its  surrounding  lamellae  constitute  a  Ha- 
versian system.  In  the  irregular  spaces  between  tbe 
Haversian  systems  of  dense  bone  are  found  discontinuous 
lamellae  also  as  a  rule  lying  in  a  general  way  parallel  to 
the  surface  of  the  mass :  these  are  known  as  interstitial 
lamellae;  and  beneath  the  outer  surface  are  always  found 
more  or  fewer  lamellae  parallel  therewnth  and  exterior  to 
the  Haversian  systems :  these  are  known  as  circumferen- 
tial lamellae. 

The  circumferential  lamellae  in  all  large  masses  of  dense 
bone  are  pierced  here  and  there  by  oblique  canals  for 
blood  vessels,  etc.,  which  communicate  with  the  Haver- 
sion  canals,  but  differ  from  them  in  having  no  surround- 
ing lamellae:  these  are  known  as  Volkmann's  canals. 
They  are  also  penetrated  transversely  by  bundles  of 
white  fibres  (which  have  undergone  calcification)  proceed- 


\ 


LAMELLATED  TISSUES.  53 

ing  from  witliout  in\vards,  atid  l)y  occasional  bundles  of 
elastic  fibres.  The  name  of  Sharpey's  fibres  is  applied  to 
both,  hut  more  especially  to  the  former.  Dense  bone  con- 
stitutes the  shafts  of  all  long  bones  and  forms  a  layer  of 
varvingf  thickness  on  the  surface  of  the  flat  and  the  short 
bones. 

In  spongy  bone  the  structure  is  more  open,  as  the  name 
implies.  The  solid  portion  consists  of  a  meshwork  of  tra- 
beculae,  or  bars  and  plates  of  varying  width  and  of  great 
irregularity  of  form,  each  of  which  is  several  lamellae  in 
thickness,  the  lamellae  being  in  a  general  way  parallel  to 
the  siirface  and  the  lacunae  communicating  therewith  by 
means  of  the  transverse  canaliculi.  The  cavities  between 
the  trabeculae,  which  are  quite  irregular  in  form  and  size, 
are  known  as  Haversian  spaces.  Spongy  bone  fills  the 
interior  of  the  short  and  the  flat  bones  and  the  ends  of 
the  long  bones. 

Just  arc  cartilage  tissue  is  found  in  masses  of  definite 
form  and  function  (organs),  which  are  known  as  cartil- 
ages, so  dense  and  spongy  osseous  tissues  are  combined  in 
the  definite  masses  which,  with  certain  associated  tissues, 
make  up  the  organs  well  known  as  bones.  As  in  the  case 
of  the  cartilages  just  referred  to,  so  in  this  instance  it  will 
be  expedient  to  describe  the  organs  in  connection  with  the 
tissues  of  which  they  are  chiefl\'  composed. 

A  bone  may  be  defined  as  a  mass  of  osseous  tissue  sur- 
rounded by  a  proper  investing  and  genetic  membrane 
known  as  the  periosteum;  permeated  by  blood  vessels  and 
lymphatics :  and  containing  the  tissue  called  marrow. 

/ 


54  PART    I.      THE    TISSUES. 

The  periosteum,  as  found  upon  the  surface  of  the  bones 
of  adult  and  particularly  of  elderly  individuals,  is  a  thin, 
tough,  closelv  felted  membrane  which  adheres  tenaciously 
to  the  structure  beneath ;  it  can,  nevertheless,  be  in  most 
cases  resolved  into  two  distinct  layers.  In  adolescence, 
childhood,  and  notably  in  foetal  life  these  are  clearly  dis- 
tinguishable. The  outer  or  fibrous  layer  has  the  structure 
of  an  ordinary  membrane,  the  constituent  bundles  (chiefly 
of  white  fibres)  being  clo^lv^inJ:er woven,  and  the  fixed 
corpuscles  flattened,  with  flattened  nuclei :  here  and  there 
occasional  fat^cells  may  be  seen,  with  numerous  blood  ves- 
sels and  lymphatics,  as  well  as  delicate  nerves :  the  outer 
surface  frequently  exhibits  extensive  areas  having  an  en- 
dothelioid  investment. 

The  inner  or  osteogenetic  layer  is  more  loosely  felted, 
and  contains  a  larger  proportion  of  elastic  fibres :  toward 
the  surface  of  the  osseous  tissue  it  contains  numerous  cor- 
puscles which  (particularly  in  young  bone)  are  larger  and 
more  irregular  in  form  than  those  of  the  outer  layer ;  the 
protoplasm  is  granular  and  the  nuclei  are  spheroidal :  these 
are  know  as  osteoblasts,  and  in  youth  are  directly  con- 
cerned in  the  formation  of  new  bone,  as  their  name  and 
that  of  the  layer  which  includes  them  implies.  In  adult 
bone  they  become  flattened  and  inactive,  forming  a  layer 
just  without  the  osseous  tissue. 

The  complex  tissue  known  as  marrow  fills  the  internal 
cavities  of  all  bones  :  those  in  the  shafts  of  the  long  bones 
are  filled  with  yellow  marrow ;  the  Haversian  spaces  of 
spongy  bone,  in  most  places  where  it  occurs,   with  red 


CHAPTER     V.      LAMELLATED    TISSUES.  55 

marrow  :  the  larger  Haversian  eanals  on  the  inner  side  of 
dense  bone  are  lined  with  a  layer  of  modified  marrow, 
which  is  proronged  in  the  smaller  canals  by  a  peculiar  form 
of  connective  tissue  cont£iining  osteoblasts,  and  finally 
continuous  on  the  outer  surface  with  the  osteogenetic 
layer  of  the  periosteum  ;  the  marrow,  as  will  be  shown 
later,  being  originally  derived  from  an  ingrowth  of  that 
layer.  The  two  kinds  of  marrow  are  closely  related  in 
structure,  the  yellow  being  derived  from  the  modification 
of  the  red  :  the  latter  will  therefore  be  described  first. 

The  basis  of  structure  of  red  marrow  is  a  delicate  frame- 
work of  retiform  tissue  associated  with  an  abundant  vas- 
cular network,  which  will  be  described  more  fully  in  an- 
other connection:  the  interstices  contain  corpuscles  of  at 
least  three  different  kinds.  Throughout  the  mass,  and 
particularly  near  the  surface,  are  the  marrow  cells,  prop- 
erly so  called  :  these  are  relatively  large,  with  faintly  gran- 
ular protoplasm  and  oval  nuclei ;  those  upon  the  surface 
of  the  mass  have  the  appearance  and  perform  the  function 
of  osteoblasts  in  young  bone.  The  interior  of  the  marrow 
contains  numerous  smaller  cells  with  granular  nuclei,  to 
which  the  name  of  erythroblasts  has  been  given,  on  ac- 
count of  the  part  they  play  in  the  formation  of  colored 
blood  corpuscles:  associated  with  these  are  great  num- 
bers of  the  immature  blood  corpuscles  themselves ;  to  these 
the  color  of  the  marrow  is  largely  due. 

Upon  the  surface  and  more  rarely  in  the  interior  of  the 
marrow  are  found  occasional  masses  of  protoplasm,  sev- 
eral times  larger  than  the  ordinary  marrow  cells,  best 
known  as  giant  cells:  they  were  called  myeloplaxes  by 


V 

56  PART  L     THE  TISSUES. 

Robin,  who  believed  them  to  be  peculiar  to  marrow  ;  but 
bodies  to  all  appearances  similar  to  them  have  since  been 
found  in  other  tissues.  It  was  proposed  by  Kolliker  to 
call  them  osteoclasts,  from  the  belief  that  they  were  agents 
in  the  absorption  of  bone;  where,  as  sometimes  occurs  in 
embryonic  bone,  the  marrow  lies  in  contact  with  tempor- 
ary cartilage  which  has  begun  to  be  absorbed,  it  has  been 
proposed  to  call  them  chondroclasts  for  a  similar  reason  : 
this  view  of  their  function  is  supported  by  the  fact  that  they 
not  infrequently  (but  by  no  means  invariably)  lie  in  little 
pits  or  depressions  of  the  surface  of  the  bone  or  cartilage 
undergoing  absorption  :  these  pits  have  been  called  How- 
ship's  lacunae  or  foveolae.  Nothing,  however,  has  yet  been 
certainly  proven  concerning  the  function  of  these  bodies : 
the  name  first  given  is  therefore  for  the  present  at  least  the 
most  desirable. 

The  giant  cells  are  of  two  kinds,  differing  chiefly  as  to 
their  nuclei.  Some  show  a  large  number  of  these  bodies ; 
others  have  but  a  single  nucleus,  which  is  always  very 
large,  and  frequently  of  an  exceedingly  irregular  shape :  the 
former  are  termed  multinuclearand  the  latter  uninuclear 
giant  cells.  It  has  been  supposed  that  the  former  are 
derived  from  the  latter  by  the  fragmentation  of  the  large 
irregular  nucleus;  but  Howell  has  shown  that  this  view 
is  certainly  very  doubtful  and  probably  erroneous. 

From  the  description  given  it  will  be  seen  that  the  red 
marrow  is  a  very  important  tissue  of  quite  complex  func- 
tion, having  important  relations  not  only  to  the  surround- 
ing bone,  but  also  to  the  elaboration  of  the  blood.  The 
yellow  marrow,  on  the  other  hand,  is,  as  far  as  our  pres- 


CHAPTKK    V.      LA.MHLLATEI)  TISSUES.  57 

ent  knowledge  u^oes,  one  of  the  most  pjissive  tissues  ot  the 
hodv.  It  differs  from  the  retl  marrow,  from  which  it  is  de- 
rived, chietly  by  the  facts  that  theerythroblastsand  imma- 
ture blood  corpuscles  arealike  wantinj^,  and  that  the  great 
majority  of  marrow  cells  in  the  interior  ol  the  mass  have 
undergone  fatty  transformation.  Upon  and  near  the 
surface  both  marrow  cells  and  giant  cells  are  found. 

From  the  description  above  given  it  will  be  seen  that 
the  nutrition  of  dense  bone  is  maintained  by  the  vessels  of 
the  ])eriosteum  directly  for  the  corpuscles  of  circumferen- 
tial lamellae,  and  by  the  vessels  of  the  Haversian  canals 
for  the  concentric  lamellae,  and  thus  indirectly  by  the  peri- 
osteum, from  which  these  vessels  are  derived  ;  the  perips- 
teum  is  thus  seen  to  be  ndt  only  a  protective,  but  also  a 
nutrient  investing  membrane:  this  is  conspicuously 
shown  by  the  fact  that  whenever  the  ])eriosteum  is  re- 
moved by  accident  or  disease  from  any  considerable  area 
of  bone  the  subjacent  osseous  tissue  perishes.  In  like  man- 
ner the  corpuscles  of  lamellae  which  make  up  the  spicules 
and  trabeculaeof  s_pongy  bone  depend  upon  the  blood 
vessels  of  the  ad[acent  marrow  for  their  food  supply,  the 
Haversian  spaces  occupying  the  same  relation  to  them  as 
the  Haversian  canals  to  the  concentric  lamellae  of  the 
dense  bone :  these  spaces  are  in  a  certain  sense  the  expan- 
sions of  the  canals,  as  the  niiirrow  which  they  contain  is 
the  continuation  of  their  lining,  and  thus,  in  a  roundabout 
way,  of  the  inner  la\'erof  the  periosteum.  It  is  also  to  be 
noted  that  the  lamellae  in  each  case  depend  for  their  nutri- 
tion upon  supplies  drawn  from  a  surface  to  which  thev are 


58  PART  I.      THE  TISSUES. 

parallel :  the  importance  of  the  transverse  canaliculi  and 
the  reason  for  their  number  and  extent  thus  becomes  evi- 
dent. 

In  speaking  of  tendon  tissue  it  was  stated  that  while  a 
mechanism  of  interstitial  increase  was  known  to  exist,  we 
had  no  clear  evidence  of  any  continuous  process  of  inter- 
stitial removal  and  replacement.  The  case  of  bone  is  dif- 
ferent :  no  one  can  examine  a  transverse  section  ol  dense 
bone,  and  note  the  manner  in  which  what  appear  to  be  re- 
cent Haversian  systems  cut  into  the  territory  of  what  are 
probably  older,  and  how  both  the  interstitial  and  the  cir- 
cumferential lamellae  are  interrupted  by  both,  without 
the  conviction  that  a  process  of  replacement  is  involved: 
and  this  conviction  is  confirmed  when  we  compare  the 
cross  section  of  the  femur  (for  example)  of  a  child  with 
that  of  an  adult,  into  the  central  marrow  space  of  which 
it  could  be  thrust,  each  with  its  Haversian  systems,  its  in- 
terstitial and  its  circumferential  lamellae.  Dense  bone 
shows  us,  however,  no  tissue  in  process  of  removal,  and 
no  Haversian  systems  in  process  of  formation :  and  the 
means  by  which  both  removal  and  replacement  take  place 
are  yet  to  be  discovered. 


CIlArTKK    VI.      OSSIFICATION.  59 


CHAPTER  VI. 

OSSIFICATION 


Attention  has  already  been  called  to  the  fact  that  the 
periosteum  is  at  once  an  investing  and  nutrient,  and  a  gen- 
etic membrane;  and  to  the  terms  applied  to  its  inner  layer 
and  the  corpuscles  contained  therein.  It  has  also  been 
pointed  out  that  the  marrow  cells  upon  and  near  the  sur- 
face of  the  red  marrow  agree  with  osteoblasts  in  form  and 
also  in  function.  The  function  of  an  osteoblast  may  now 
be  briefly  stated  by  saying  that  it  is  its  normal  destiny  to 
become  a  bone  corpuscle.  Bone  is  laid  down  in  lamellae  by 
the  activitv,  j)rol)ablv  periodical  in  character,  of  an  osteo- 
genetic  layer,  the  osteoblasts  nearest  the  lamella  just  pre- 
viouslv  formed  being  enclosed  by  the  new  layer  of  fibril- 
lated  and  calcified  matrix,  and  thus  converted  into  bone 
cor])uscles. 

It  has  already  been  stated  and  should  always  be  born 
ill  mind  that  the  marrow,  as  will  shortly  be  seen,  is  a  de- 
velopment from  the  osteogenetic  layer  of  the  periosteum: 
its  outer  surface,  with  its  layer  of  osteoblasts,  is  as  truly 
an  ostcojrenetic  laver  as  that  from  which  it  is  derived : 
attempts  have,  indeed,  been  made  to  define  it  as  a  distinct 
membrane  under  the  name  of  the  endosteum;  but  this  is 
unnecessary:  and  is  not  advisable,  if  for  no  other  reason. 


60  PART    I.      THE    TISSUES. 

on  account  of  th^  fact  that  it  cannot  bs  anatomically  sep- 
arated from  the  tissue  below,  nor  its  boundary  on  that 
side  at  all  clearly  defined. 

The  formation  of  osseous  tissue  by  the  enclosure  of  os- 
teoblasts in  a  fibrillated  and  calcified  matrix  is  called  ossi- 
fication, wherever  it  occurs,  as,  for  instance,  in  the  bones 
of  a  growing  child  or  young  animal.  The  term  is  more 
especialh'  applied,  however,  to  the  original  formation  and 
early  development  of  bone  as  it  takes  place  in  the  embryo. 
Used  in  this  sense,  the  name  is  applied  to  two  processes 
often  regarded  as  essential!}'  different,  and  designated  re- 
spectiveh--  as  intramembranous  ossification,  or  the  devel- 
opment of  bone  in  previously  existing  connective  tissue, 
and  ossification  in  cartilage,  or  the  development  of  bone 
in  previousl}^  existing  cartilage.  It  is  of  the  highest  im- 
portance, how^ever,  to  keep  clearl}'  in  mind  the  fact  just 
stated,  that  bone  is  always  formed  in  a  peculiar  kind  of 
connective  tissue,  the  osteogenetic  layer  already  defined  as 
in  every  instance  derived  directly  or  indirectly  from  a  mem- 
brane. Bone  is  therefore  alwa^'s  derived  from  the  modifi- 
cation of  a  membrane:  and  the  facts  of  comparative  anat- 
omy and  of  pathology,  on  the  other  hand,  show  that 
almost  any  membrane  may  under  certain  circumstances 
become  osteogenetic. 

The  so-called  ossification  in  cartilage,  or,  as  it  is  often 
termed  (and  the  expression  is  still  more  apt  to  mislead ), 
the  ossification  of  cartilage,  is  in  reality  the  replacement 
of  cartilage  by  bone,  the  cartilage  itself  being  absorbed 
and  disappearing  in  great  measure  before  bone  forma- 
tion takes  place.     It  is  particularly  important  to  avoid 


lA^' 


CHAI'TI;K    VI.      OSSIFICATION.  <)1 

the  en oiicous  impression  that  is  sometimes  caused  by  the 
exjjression  iti  (|uestion,  to  the  effect  that  the  previously  ex- 
istinjj  cartilage  is  in  some  manner  transformed  into  hone: 
it"  this  l)c  borne  constantly  in  mind,  there  is  no  objection 
to  the  use  ot  the  term  in  either  form  :  nor,  with  this  quali- 
fication, to  the  use  of  the  terms  "membrane  bone"  and 
''cartilage  bone,"  frequently  employed  for  convenience  to 
desi<j:nate  briefly  bones  formed  respectively  in  the  two 
methods  above  defined. 

Since  bone  is  to  be  regarded  as  the  result  of  a  develop- 
ment originating  in  connection  with  a  membrane,  we  may 
properlv  first  consider  the  process  of  intramembranous 
ossification :  this  takes  place  in  the  human  body  and  in 
that  of  the  higher  vertebrates  generalh'  in  a  very  small 
number  of  bones,  the  great  majority  of  those  entering  into 
the  structure  of  the  adult  skeleton  being  preceded  b}'  car- 
tilage: the  tegmental  bones  of  the  skull,  the  squamous  por- 
tion of  the  temporal  bone,  the  bones  of  the  face  and  jaws 
(excepting  a  small  portion  of  the  lower  jaw),  and  the  clav- 
icle are  the  only  bones  not  so  preceded,  or,  in  other  words, 
the  only  so  called  membrane  bones. 

A  studv  of  the  development  in  the  embryo  of  any  one 
of  these  bones  gives  substantially  the  following  results. 
The  place  of  the  future  bone  is  at  first  occupied  by  a  mass 
of  embryonic  connective  tissue  not  yet  possessing  the  den- 
sity and  structure  of  membrane,  permeated  by  a  network 
of  blood-vessels.  At  one  or  more  places,  known  as  centres 
of  ossification,  bundles  of  stout  fibres  are  formed,  radia- 
ting outward  from  a  point  midway  between  the  adjacent 


62  PART  I.     THE  TISSUES. 

vessels :  these,  which  are  known  as  OSteogenetic  fibres, 
resemble  white  fibres  in  appearance,  but  are  less  distinctly 
fibrillated  :  between  them,  and  crowded  upon  the  surfaces 
of  the  bundles,  are  large  numbers  of  connective  corpuscles, 
modified  to  form  osteoblasts.  The  homogeneous  sub- 
stance which  forms  their  matrix  now  begins  to  undergo 
calcification,  the  salts  being  at  first  deposited  in  the  form 
of  minute  globules;  as  these  become  more  and  more  nu- 
merous, they  fuse  together  forming  a  continuous  and  ap- 
parently homogenous  mass :  the  osteoblasts  imbedded 
therein  becoming  the  bone  corpuscles,  and  the  modified, 
bundles  of  fibres  becoming  spicules  of  bone. 

The  formation  of  new  fibres,  Avith  their  associated  cor- 
puscles, continues  to  go  on  in  advance  of  calcification  at 
the  extremities  of  the  spicules,  the  tufts  of  bundles  diverg- 
ing in  such  manner  as  to  anastomose  with  those  of  adja- 
cent spicules,  their  growth  taking  such  direction  as  to  pass 
between  the  meshes  of  the  vascular  network.  As  a  result^ 
there  is  formed  a  spongy  network  of  bone  interlacing  with 
the  network  of  blood  vessels,  the  interstitial  embryonic 
connective  tissue  forming  the  basis  of  the  primar^^  mar- 
row which  fills  the  spaces  and  in  which  the  blood  vessels 
are  imbedded.  While  this  is  taking  place  in  the  interior 
of  the  mass,  the  embryonic  connective  tissue  upon  its  sur- 
face becomes  converted  into  a  well  defined  la3^er  of  fibrous 
membrane,  the  fibrous  layer  of  the  periosteum :  from  the 
OSteogenetic  tissue  immediately  beneath  dense  periosteal 
bone  is  formed.  The  ossification  thus  set  up  is  continued 
Outw^ard  from  the  centre  or  centres  of  ossification  until  the 
whole  territory  involved  is  converted  into  osseous  tissue, 


CHAPTKK     VI.       OSSIFICATION.  63 

with  the  associated  periosteum,  maiTow  and  blood  ves- 
sels: as  the  botie  increases  in  thickness  new  layers  of  dense 
bone  are  deposited  by  the  periosteum,  while  that  first 
formed  is  absorbed  and  replaced  by  the  spongy  bone  of 
the  interior. 

In  the  replacement  of  cartilage  by  bone,  or,  as  it  has 
been  sometimes  termed,  intracartilaginous  ossification, 
blood  vessels  also  play  a  conspicuous  part,  as  we  shall 
presently  see;  the  centres  of  ossification  in  this,  as  in  the 
preceding  case,  arising  by  the  development  of  highly  vas- 
cular areas  of  osteogenetic  tissue :  the  formation  of  these 
areas  and  their  subsequent  extension,  known  as  the  vas- 
cularization of  cartilage,  is  the  immediate  preliminary  to 
the  deposition  of  bone.  The  process  is  rendered  ;nore 
complex  by  the  fact  that  the  previously  existing  cartilage 
must  necessarily  be  removed,  at  least  in  great  measure, 
before  bone  can  be  deposited  in  its  stead ;  and  the  addi- 
tional fact  that  the  receding  cartilage  is  itself  the  seat  of 
noteworthy  changes  which  invariably  take  place,  although 
their  direct  relation  to  the  formation  of  bone  is  by  no 
means  clear.  It  should  also  be  mentioned,  before  begin- 
ning a  detailed  discussion  of  the  process  in  question,  that 
the  replacement  proper  of  the  cartilage  is  also  accom- 
panied or  followed  at  a  very  early  stage  by  its  investment 
with  bone  formed  by  the  newly  developed  periosteum.: 
and  that  this  takes  place  at  the  outset  in  a  manner  some- 
what resembling  the  process  of  intramembranous  ossifica- 
tion just  described;  whereas,  at  a  later  stage,  the  torraa- 
tion  of  dense  bone  takes  place. 


64  PART    1.     THE    TISSUES, 

We  ma}',  therefore,  recognize  four  distinct  and  definite 
stages  in  the  process  of  intracartilaginous  ossification,  so- 
called  (to  the  second  of  which  alone,  however,  the  term  is 
strictl^v  applicable) :  these  stages  are  in  the  main  succes- 
sive for  any  particular  point;  though  all  may  be  in  pro- 
gress at  the  same  time  at  points  adjacent  to  each  other ; 
they  are  definable  as  follows. 

The  first,  which  may  be  called  the  transformation  of 
cartilage,  includes  all  those  changes  which  take  place  in 
that  tissue  from  the  first  disturbance  of  its  normal  condi- 
tion to  its  final  dissolution  in  great  measure.  The  second 
is  the  development  of  spongy  bone  in  the  spaces  formed  by 
the  dissolution  of  the  cartilage  and  upon  its  remains,  or 
the  formation  of  endochondral  bone.  The  deposition  of 
spong}^  bone  beneath  tbe  newly  developed  periosteum  as 
an 'investment  of  the  endochondral  bone  and  the  trans- 
forming cartilage  is  the  third :  from  the  position  where  it 
occurs  this  is  designated  the  formation  of  perichondral 
bone,  or,  as  it  is  sometimes  called,  primary  periosteal  bone- 
The  fourth  and  last  stage  is  the  formation  of  dense  bone 
surrounding  the  perichondral  bone,  through  the  continued 
activity  of  the  periosteum ;  the  transition  from  the  one  to 
the  other  being  in  some  cases  exceedingl}'  gradual. 

We  may  now  enter  upon  a  discussion  of  the  changes 
which  take  place  in  the  formation  of  the  shaft  of  one  of 
the  long  bones,  such  as  the  humerus,  the  tibia,  or  one  of 
the  metatarsal  bones.  The  bone  is  preformed,  to  use  a 
current  expression,  in  cartilage:  that  is  to  say,  its  future 
place  is  occupied  by  a  mass  of  hyaline  cartilage  having  in 
a  general  way  the  form  and  relations  of  the  bone   that  is 


CHAPTER    VI.       OSSIFICATION.  65 

to  replace  it :  the  mass  of  cartilage  is  covered  by  a  single 
and  closely  adhering  fibrous  layer,  the  perichondrium, 
which  is  moderately  rich  in  blood  vessels.  The  transfor- 
mation of  the  cartilage  involves  four  recognizable  changes, 
which  may  be  designated  as  the  rearrangement  of  the 
corpuscles,  the  calcification  of  the  matrix,  the  forma- 
tion of  primary  areolae,  accompanied  by  the  degenera- 
tion of  the  corpuscles,  and  the  formation  of  secondary 
areolae  by  the  partial  dissolution  of  the  matrix. 

At  a  point  near  the  centre  of  the  mass  the  corpuscles  be- 
gin to  multiply  and  to  increase  in  size,  and  (in  a  manner 
not  3'et  clearly'  understood)  to  arrange  themselves  in 
columns  or  rows  which  at  first  radiate  from  the  central 
point,  with  intervening  regions  consisting  of  matrix  only, 
thus  forming  a  spheroidal  region  of  transformation  which 
continues  to  increase:  when  the  surface  of  the  mass  is 
reached  on  the  adjacent  sides  the  region  of  transformation 
is  of  course  restricted  to  the  cartilage  lying  in  the  direc- 
tion of  the  ends :  and  as  it  advances  toward  them  in  both 
directions  the  columns  of  rearranged  corpuscles  and  the 
intervening  regions  of  matrix  soon  take  on  a  direction 
parallel  to  the  axis  of  the  bone. 

Very  shortly  after  the  rearrangement  of  the  corpuscles 
just  described,  the  deposition  of  lime  salts  in  the  matrix 
takes  place,  particularly  in  those  tracts  which  lie  between 
the  corpuscular  rows,  bars  and  plates  of  calcified  cartilage 
thus  being  formed.  At  the  same  time  the  cavities  in  which 
the  cartilage  corpuscles  lie  begin  to  be  enlarged,  while  the 
corpuscles  themselves  undergo  degeneration,  becoming 
shrunken  and  irregular  in  form,  and  lying  in  the  enlarged 


66  PART    I.      THE    TISSUES. 

cavities,  which  are  now  known  as  the  primary  areolae 
above  referred  to.  In  some  cases  the  areolae  consist  of  two 
or  three  cavities  in  the  same  row,  united  by  the  dissolution 
of  the  thin  lamella  of  matrix  lying  between  them.  The 
farther  dissolution  of  the  matrix,  and  the  formation  of  the 
secondary  areolae,  in  which  the  deposition  of  endochon- 
dral bone  begins,  depends  upon  the  development  of  other 
structures  presently'  to  be  described. 

About  the  time  that  the  transformation  of  cartilage  is 
set  up  in  the  centre  of  the  mass  important  changes  take 
place  in  the  surrounding  membrane,  beginning  in  a  zone 
immediately  adjacent,  and  proceeding  thence  toward  either 
extremit}'  simultaneously  with  the  advancing  transfor- 
mation of  the  mass  within.  The  fibrous  layer  becomes 
thicker  and  more  highly  vascular:  its  inner  portion  be- 
comes looser,  and  the  corpuscles  rapidly  increase  in  num- 
ber and  in  size,  with  associated  changes  in  form  and 
structure:  in  other  words,  the  perichondrium  of  the  em- 
bryonic cartilage  becomes  converted  into  the  periosteum 
of  the  future  bone,  with  an  outer  fibrous  and  inner  osteo- 
genetic  la^^er. 

By  the  time  the  central  spheroidal  area  of  transform- 
ing cartilage  has  reached  the  lateral  surfaces  of  the  mass, 
the  adjacent  zone  of  periosteum  is  fully  formed ;  and  the 
deposition  of  a  delicate  network  of  spongy  bone  may  have 
already  taken  place.  As  the  two  regions  of  activity  come 
in  contact,  one  or  more  loops  of  blood  vessels  grow  out 
toward  the  transformed  cartilage,  carrying  with  them  an 
investment  of  osteogenetic  tissue:  before  their  advance  the 
transformed  cartilage  is  absorbed,  and  they  quickly  reach 


CHAPTHR    VI.       OSSIKICATION.  07 

the  centre  of  the  mass.  This  is  sometimes  termed  the 
primary  vascular  invasion.  From  the  centre  the  forma- 
tion of  vascular  loops  and  the  associated  development  of 
the  investing  osteogenetic  tissue  is  directed  toward  the 
ends  of  the  bone,  following  the  advancing  areas  of  trans- 
formation of  cartilage ;  the  thin  lamellae  of  matrix  lying 
between  the  primary  areolae  in  the  longitudinal  rows  are 
aljsorbed,  as  well  as  (to  some  extent)  those  lying  between 
adjacent  rows:  there  is  thus  formed  a  network  of  elon- 
gated spaces,  the  secondary  areolae,  separated  by  inter- 
communicating bars  and  plates  (trabeculae)  of  calcified 
cartilage  matrix,  and  opening  toward  the  centre  of  the 
bone. 

These  spaces  are  rapidly  filled  by  the  advancing  blood 
vessels  and  associated  osteogenetic  tissue,  which  together 
form  the  primary  marrow:  the  surface  of  the  mass  is  in- 
vested with  numerous  osteoblasts,  which  are  brought 
directly  in  contact  with  the  trabeculae  of  calcified  cartilage 
matrix :  upon  the  surfaces  of  the  latter  the  deposition  of 
thin  lamellae  of  true  bone  substance  begins.  There  is  thus 
formed  a  spongy  mass  of  bone  trabeculae  having  a  small 
quantit}'  of  calcified  cartilage  included  in  their  structure, 
and  forming  an  area  which  advances  towards  the  end  of 
the  l)ody  just  behind  the  area  in  which  the  last  steps  in 
the  transformation  of  the  cartilage  are  taking  place:  this 
is  the  endochondral  bone.  In  the  long  bones  with  hol- 
low shafts  it  is  itself  a  more  or  less  temporary  structure, 
being  absorbed  from  behind  almost  as  rapidly  as  it  is 
formed  along  the  area  of  advance,  its  place  beingtaken  by 
the  permanent  marrow.     This  is  formed  from  the  modifi- 


68  PART  I.      THE  TISSUES. 

cation  of  the  primary  marrow,  which  at  first  consists 
only  of  the  blood  vessels  and  the  associated  osteogen- 
etic  tissue,  which  is  quite  delicate  and  almost  gelatinous 
in  structure :  later,  its  retiform  framework  becomes  more 
fulh'  developed,  giant  cells  and  erythroblasts  appear,  and 
the  characters  of  ordinary  red  marrow^  are  assumed;  these 
in  turn  (in  the  region  referred  to)  giving  place  to  those  of 
the  yellow  marrow  which  finally  occupies  the  cavity. 

It  has  already  been  stated  that  the  newly  developed  zone 
of  periosteum  begins  shortly  after  its  formation  to  de- 
posit a  layer  of  spongy  bone  upon  the  surface  of  the  trans- 
forming cartilage  in  the  middle  of  theshaft,  similar  to  that 
first  formed  where  the  seat  of  ossification  is  a  membrane. 
Along  with  the  progress  of  the  areas  of  the  transforming 
cartilage  toward  the  extremities  of  the  bone,  and  the  im- 
mediately successive  development  of  endochondral  bone  in 
the  interior  of  the  mass,  there  is  a  corresponding  advance 
in  both  directions  of  the  change  which  converts  the  fibrous 
membrane  investing  the  cartilage  into  periosteum.  This, 
again,  is  immediately  followed  by  the  deposition  of  a  layer 
of  spongy  bone,  or,  rather,  by  the  extension  of  the  layer 
already  begun.  While  the  layer  so  formed  is  in  this  man- 
ner steadily  increasing  in  extent,  it  also  undergoes  increase 
in  thickness  in  the  region  where  it  was  first  formed. 
There  is  thus  formed  a  sheath  of  spongy  bone,  shaped  like 
a  dice  box  open  at  both  ends,  thickest  in  the  middle  and 
gradually  becoming  thinner  towards  its  extremities,  which 
contains  in  its  middle  the  newly  formed  permanent  mar- 
row :  its  ends  surround  the  masses  of  endochondral  bone 
previously  described ;  immediatel}'  beyond  lies  the  area  of 


CHAPTER   VI.      OSSIFICATION.  69 

transforminjj^  cartila<i^c:  and  still  beyond  (and  invested  as 
yet  by  perichondrium)  the  hyalinecartilage  which  has  thus 
far  underg^one  no  chan<ye.  This  sheath  of  spongy  bone  is 
the  perichondral  bone,  or  as  it  is  sometimes  called,  the 
primary  periosteal  bone. 

As  the  periosteum  extends  its  borders  toward  the  ends 
of  the  future  bone,  its  newly  formed  osteogenetic  la3'er 
making  the  deposit  of  perichondral  bone  just  described,  a 
change,  more  or  less  gradual,  takes  place  in  the  activity  of 
its  central  and  older  portion.  The  meshes  of  the  vascular 
network  within  its  inner  layer  become  more  and  more 
elongated  in  the  direction  of  the  long  axis  of  the  bone : 
the  osteogenetic  tissue  which  surrounds  the  vessels,  instead 
of  continuing  to  occupy  a  large  portion  of  the  intervascu- 
lar  spaces  as  marrow,  deposits  successive  lamellae  of  bone 
substance  concentric  to  the  vessels,  until  the  spaces  are  re- 
duced to  slender  canals.  Haversian  systems  are  thus 
developed;  while  from  the  immediate  surface  of  the  perios- 
teum interstitial  and  circumferential  lamellae  are  pro- 
duced, and  the  sheath  of  perichondral  bone  is  itself 
invested  with  a  layer  of  dense  bone,  or  permanent  peri- 
osteal bone.  The  perichondral  bone  is  but  little  more  en- 
during than  the  endochondral  bone  which  it  for  a  time 
surrounds :  it  has  already  been  stated  that  as  the  latter 
advances  toward  the  degenerating  cartilage,  it  is  itself,  in 
the  long  bones,  sooner  or  later  absorbed  and  replaced  bv 
the  permanent  marrow :  the  perichondral  bone  suffers  a 
like  fate,  the  mass  of  marrow  becoming  enlarged  in  diameter 
as  well  as  in  length,  and  the  surrounding  spongy  bone  being 
absorbed  about  it  until  the  marrow  lies  directly  beneath 


70  PART  I.      THE    TISSUES. 

the  tube  of  dense  bone  which  forms  the  permanent  shaft 
of  the  femur  or  tibia,  as  the  case  may  be. 

It  will  be  remembered  that  the  mass  of  cartilage  which 
in  the  foetus  represents  one  of  the  long  bones  under  con- 
sideration, is  very  much  smaller  than  the  bone  which  we 
find  in  its  place  even  in  the  newborn  child :  the  increase  in 
thickness  will  be  readily  understood.  It  should  be  borne 
in  mind,  however,  that  this  increase  in  thickness  is  for 
quite  a  while  due  to  the  formation  of  bone  of  the  peri- 
chondral type  only :  it  is  not  until  some  months  after  birth 
that  dense  bone  of  the  ordinary  type  is  deposited,  the 
shafts  of  the  long  bones  remaining  up  to  that  time  more 
or  less  spongy  throughout. 

The  increase  in  length  is  due  to  a  continuous  growth  of 
the  primary  cartilaginous  mass  itself,  which  up  to  birth 
and  for  some  time  after  continues  to  increase  in  length  and 
also  in  thickness  in  the  part  which  has  not  as  yet  begun 
to  undergo  transformation.  This  growth  continues  at  a 
constantly  decreasing  rate,  the  region  of  untransformed 
cartilage  thus  becoming  smaller  and  smaller.  It  is  en- 
croached upon  alike  by  the  advancing  endochondral  bone 
of  the  shaft,  and  the  similarlj^  formed  but  more  permanent 
mass  of  spongy  bone  in  the  epiphyssis,  until  finally  the  two 
areas  of  ossification  meet,  and  the  union  of  the  epiphysis 
with  the  shaft  is  accomplished,  the  two  masses  of  spongy 
bone  and  their  investing  layers  of  dense  bone  alike  becom- 
ing confluent. 

The  table  on  the  opposite  page  presents  a  summary  of 
the  various  processes  which  have  been  described  above  as 
taking  place  in  the  formation  of  the  shaft  of  a  long  bone. 


REPLACEMENT  OF  CARTILAGE  TO  FORM  THE  SHAFT  OF  A 
LONG  DONE  (c.  g.  FEMUR.) 


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72  PaUT    I.     THE   TISSUES. 

In  the  short  bones,  and  in  the  epiphyses  of  the  longbones, 
the  replacement  of  cartilage  takes  place  in  an  essentially 
similar  manner,  the  vascular  invasions  reaching  the  cen- 
tre of  the  mass  of  transforming  cartilage,  and  the  develop- 
ment of  spongy  endochondral  bone  taking  place  on  all 
sides.  Some  of  the  trabeculae  thus  formed  are  shortly 
afterwards  absorbed,  while  others  become  larger,  and 
form  part  of  the  permanent  framework  of  the  interior  of 
the  bone.  It  is  probable  even  here,  however,  that  there  is 
in  the  earlier  part  of  adult  life  at  least  more  or  less  of  ab- 
sorption and  rebuilding  continuously  going  on.  An  invest- 
ing layer  of  perichondral  bone  is  formed,  and  contributes 
to  the  permanent  spongy  mass :  and  later  a  thin  layer  of 
dense  bone  is  superimposed.  There  are  some  very  inter- 
esting variations  in  detail  as  to  the  manner  in  which  ossifi- 
cation arises  in  some  of  the  bones,  but  their  consideration 
would  be  foreign  to  the  purpose  of  this  work. 


CHAPTER    VII.      THE    BLOOD^  73 


CHAPTER   VII 
THE   BLOOD, 


The  blood  consists  of  a  fluid  portion,  the  plasma,  and 
numerous  corpuscles  found  floating  therein.  It  has  some- 
times been  described  as  a  tissue  with  a  fluid  matrix.  It  is 
perhaps  a  straining  of  the  use  of  these  terms  to  apply 
them  to  the  blood :  it  is  certain,  however,  that  a  descrip- 
tion of  the  structural  elements  of  the  body  would  be  in- 
complete without  an  account  of  those  contained  in  that 
fluid.  While  they  cannot  be  strictly  defined  as  connective 
tissue  elements,  their  origin  is  such  as  to  warrant  their  de- 
scription at  this  time. 

The  plasma  is  during  life  and  in  health  a  perfect  fluid  as 
long  as  it  is  contained  in  the  blood  vessels.  When  allowed 
to  escape  therefrom,  or  more  rarely  under  certain  abnor- 
mal conditions  while  contained  therein,  it  undergoes  coag- 
ulation, an  important  process,  which  concerns  us  chiefly 
because  of  the  formation  of  a  solid  substance  whose  ap- 
pearance should  be  familiar  to  the  student  of  histology. 
It  is  known  as  fibrin,  and  when  present  in  small  quanti- 
ties can  be  plainly  seen  to  consist  of  exceedingly'  delicate 
filaments  interlacing  in  every  direction ;  as  the  quantity 
increases,  the  meshes  of  the  network  thus  formed  become 
filled  with  fibres,  until  a  practically  continuous  solid  mass 


74  PART    I.      THE    TISSUES. 

is  formed :  when  hardened  and  stained  such  a  mass  pre- 
sents in  section  a  granular  appearance  due  to  the  cross- 
section  of  the  fibres. 

The  corpuscular  elements  contained  in  the  plasma  and 
constituting  normally  a  little  over  one-third  of  the  vol- 
ume of  the  blood,  are  chiefly  of  two  kinds,  the  colored  (or 
so-called  red)  corpuscles,  or,  as  they  have  been  termed, 
the  erythrocytes,  and  the  colorless  (or  so-called  white) 
corpuscles,  which  are  also  commonly  known  as  leuco- 
cytes. There  are  also  present  in  normal  blood  smaller  or 
larger  quantities  of  minute  bodies  known  by  various 
names,  the  most  common  of  which  is  the  blood  platelets. 

The  colored  corpuscles  are  by  far  the  most  numerous, 
about  five  millions  being  contained  in  a  cubic  millimetre  of 
the  blood  of  a  healthy  man :  the  number  in  the  blood  of 
women  is  about  ten  per  cent,  less :  in  certain  forms  of  dis- 
ease their  number  may  be  very  greatly  reduced.  When 
seen  singly  under  the  microscope  each  colored  corpuscle  is 
seen  to  be  of  a  yellowish  green  color,  the  former  tint  pre- 
vailing in  arterial,  the  latter  in  venous  blood.  The  form 
of  each  is  that  of  a  biconcave  circular  disk,  the  central 
portion  being  slightly  hollowed  on  each  side  and  the  mar- 
gin rounded. 

The  size  of  the  colored  corpuscles  varies  within  rather 
wide  limits,  the  same  sample  of  human  blood  exhibiting 
individual  corpuscles  whose  transverse  diameter  is  as 
great  as  ten  micra,  and  others  that  are  but  half  as  broad : 
by  far  the  great  majority,  however,  will  be  found  to  be 


CHAPTER    VII.      THE    BLOOD.  7.' 

from  seven  to  eight  niicra  in  breadth :  the  average  meas- 
urement has  been  variously  stated  by  different  observers 
after  a  very  large  number  of  measurements ;  it  is  not  far 
from  the  truth  to  say  that  it  is  about  seven  and  three- 
fourths  micra.  Little  diflerence,  if  any,  is  found  in  this 
respect  in  the  blood  of  persons  of  different  age  or  sex,  nor 
is  any  noticeable  in  that  of  the  various  races  of  mankind. 
The  greatest  thickness  of  the  blood  corpuscle  is  about 
one-fourth  its  breadth.  The  smallest  corpuscles  found  are 
sometimes  distinguished  as  a  separate  form  of  elements 
under  the  name  of  microcy tes ;  but  this  distinction  is  of 
doubtful  significance. 

« 

The  colored  corpuscles  of  the  blood  of  all  mammals  save 
those  of  the  camel  family  resemble  those  of  human  blood 
in  form  and  general  appearance:  they  vary,  however, 
greatly  in  size  in  different  species,  the  differences  that  are 
found  to  occur  having  no  relation  to  the  differences  in  size 
of  the  animals  themselves  ;  those  of  the  mouse,  for  exam- 
ple, being  distinctly  larger  than  those  of  the  horse.  The 
largest  known  are  those  of  the  elephant  (between  nine  and 
ten  micra  in  diameter)  and  the  smallest  those  of  the  musk 
deer  (about  two  and  one-half  micra).  There  is,  also,  no 
necessary  close  resemblance  in  this  respect  between  nearly 
allied  animals ;  thus,  while  the  colored  corpuscles  of  the 
blood  of  oxen,  sheep  and  goats  are  very  nearly  of  the 
same  size,  those  of  dogs  and  cats  differ  greatly. 

While  the  colored  corpuscles  of  the  blood  of  many  mam- 
mals differ  in  size  so  decidedly  from  those  of  human  blood 
asto  make  their  distinction  a  matter  of  absolute  certainty, 
there  are  several  species  of  mammals  whose  colored  blood 


76  PART    I.      THE    TISSUES. 

corpuscles  approximate  so  closely  in  size  to  those  of  man 
as  to  render  impossible  a  certain  distinction  between  them. 
While,  therefore,  it  might  be  possible  under  certain  circum- 
stances to  testify  that  a  given  stain  was  not  caused  by 
human  blood,  the  converse  is  not  justifiable  in  the  present 
state  of  our  knowledge. 

The  structure  of  the  colored  corpuscles  has  been  and  still 
is  a  matter  of  much  debate.  By  some  the  disk  is  regarded 
as  consisting  of  a  denser  colorless  portion,  spongy  in  struct- 
ure, the  stroma,  whose  meshes  contain  the  fluid  colored 
portion,  a  solution  of  the  substance  known  as  haemoglob- 
in: its  behavior  towards  certain  reagents  (notably  to- 
ward water)  leads  others  to  the  conviction  that  what  we 
have  is  realh^  a  closed  sac  (or  cell  in  the  true  sense  of  the 
word)  filled  with  the  colored  fluid.  It  is  at  least  certain 
that  the  corpuscle  is  far  from  being  a  mass  of  protoplasm, 
being  as  greatly  modified  therefrom  as  a  fat  cell.  It  should 
also  be  noted  that  a  nucleus  is  wanting  in  the  ordinary 
colored  blood  corpuscles  of  all  mammalia:  in  which  re- 
spect they  differ  from  those  of  all  other  vertebrates. 

When  blood  is  drawn  from  the  vesvsels,  or  when,  from 
any  cause,  it  stagnates  for  a  time  within  them,  the  colored 
corpuscles  show  a  tendency  to  adhere  together  by  their 
sides  in  masses  resembling  piles  of  coin  ;  this  formation  of 
rouleaux,  as  it  is  termed,  was  once  supposed  to  be  pecul- 
iar to  blood  corpuscles;  but  it  has  been  shown  that  under 
certain  circumstances  the  same  process  may  be  caused  to 
take  place  with  floating  disks  of  cork  or  other  substances 
under  conditions  that  are  purely  mechanical.  The  rou- 
leaux thus  formed  not  unfrequently  arrange  themselves  in 


CHAPTER    VII.      THK    lU.OOD.  7< 

a  coarse  network  in  whose  meshes  the  colorless  corpus- 
cles may  be  seen,  and  the  first  formed  filaments  of  fibrin 
detected  as  coaj^ulation  begins.  The  formation  of  rou- 
leaux does  not  take  ])laee  in  the  blood  of  vertebrates  other 
than  mammals,  the  thickening  of  the  centre  of  the  cor- 
puscles due  to  the  presence  of  the  nucleus  acting  as  an 
obstacle  thereto. 

If  sodium  chloride  or  any  other  salt,  sugar,  glycerine,  or 
any  other  reagent  that  tends  to  increase  the  specific  grav- 
ity of  the  plasma  be  added  to  fresh  blood,  or  if  the  same 
effect  is  produced  by  circumstances  favoring  rapid  evapo- 
ration, the  colored  corpuscles  undergo  a  characteristic 
change  of  form  known  as  crenation,  the  corpuscle  becom- 
ing shrivelled  and  the  surface  studded  with  minute  projec- 
tions, the  whole  bcingin  appearance  not  unlike  the  fruit  of 
a  thorn-ap])le  or  a  horse-chestnut.  This  change  may  pos- 
sibly take  place  in  the  vessels  themselves  under  circum- 
stances temporarily  causing  an  appreciable  difference  in 
the  proportion  of  water  in  the  blood. 

The  colorless  corpuscles,  or  leucocytes,  are  nucleated 
masses  of  protoplasm  which  may  be  regarded  as  tvpical 
animal  cells.  When  at  rest  they  are  s])heroidal  in  form, 
but,  as  will  be  seen  later,  are  capable  of  very  great  modi- 
fication in  this  respect.  The}'  are  far  less  numerous  than 
the  colored,  a  cubic  millimetre  of  healthy  human  blood 
containing  on  an  average  about  ten  thousand  :  the  number 
varies  greatly,  however,  not  only  in  different  individuals, 
but  in  the  same  individual  under  different  conditions; 
thus,  the  number  is  greatly  increased  shortly  after  eating 


78  PART  I.     THE  TISSUES. 

and  marked!}'  diminished  by  prolonged  fasting:  we  may 
therefore  find  in  the  blood  of  a  healthy  person  at  one  time 
twice  as  many  to  the  cubic  millimetre,  at  another  even  less 
than  half  as  many  as  the  average  above  given.  Since  the 
number  of  the  colored  corpuscles  is  not  subject  to  such 
marked  fluctuation,  the  ratio  between  the  two  varies 
therewith  ;  and  the  number  of  colorless  corpuscles  is  often 
stated  in  terms  of  that  ratio  as  ranging  between  one  col- 
orless to  two  hundred  and  fifty  colored  and  one  colorless 
to  over  one  thousand  colored  corpuscles:  the  average 
being  about  one  colorless  to  five  hundred  colored  corpus- 
cles. This  mode  of  statement  is,  however,  unsatisfactory, 
for  the  reason  that,  as  will  be  readily  seen,  a  like  change 
of  ratio  would  result  from  a  marked  diminution  in  the 
number  of  colored  corpuscles  or  an  undue  increase  in  the 
number  of  the  colorless,  changes  in  each  case  of  very  great 
importance,  but  of  quite  dift'erent  significance. 

The  colorless  corpuscles  vary  also  in  size  to  a  great  ex- 
tent: their  average  diameter,  when  in  the  spheroidal 
state,  may  be  stated  as  about  ten  micra.  The  smallest, 
sometimes  distinguished  specifically  as  small  leucocytes, 
have  large  spheroidal  nuclei,  surrounded  by  a  small 
amount  of  protoplasm :  they  may  be  regarded  as  newly 
formed  elements  that  have  only  recently  entered  the  blood 
stream  ;  they  are  somewhat  smaller  than  the  average  col- 
ored corpuscles.  When  fully  developed  they  may  become 
as  much  as  twice  as  large,  the  increase  being  chiefly  in  the 
amount  of  protoplasm :  they  are  then  sometimes  termed 
large  leucocytes;  still  retaining  the  single  spherical  nu- 
cleus. 


CHAPTER    YH.      THK    BI.OOD.  70 

The  smaller  and  lar<?cr  uninuclear  elements  make  uj)  in 
all  about  tvvciity-fivc  per  cent,  of  the  leucocytes  of  the 
blood.  Almost  all  of  the  remainder  (about  seventy  per 
cent,  ot  the  whole  number)  are  what  are  sometimes 
termed  multinuclear  leucocytes:  these,  which  are  com- 
paratively uniform  in  size  and  somewhat  larger  than  the 
colored  corpuscles,  have  either  two  or  three  small  nuclei 
or  one  lart^^e  nucleus  of  irregular  form  and  apparently 
about  to  undergo  fragmentation.  In  addition  there  may 
be  found  (occurring  but  sparsely  in  normal  blood)  a  small 
number  of  leucocytes  with  pale  nuclei  and  highly  granular 
protoplasm,  the  contained  granules  staining  deeply  with 
eosin  and  similar  reagents.  These  are  sometimes  known 
as  granular  or  (from  their  affinity  for  the  substance  just 
mentioned)  eosinophilous  leucocytes. 

When  human  blood  or  that  of  any  manmal  is  examined 
at  an  ordinary  temperature  the  colorless  corpuscles  are 
spheroidal  and  motionless :  if,  however,  the  stage  of  the 
microscope  be  so  heated  as  to  maintain  the  temperature 
of  the  blood  at  that  of  the  living  body,  the  spheroidal 
form  is  no  longer  maintained.  The  living  corpuscle  puts 
out  one  or  more  stout  lobular  processes,  thus  assuming  an 
irregular  form  bounded  by  sweeping  curves  :  these  curves 
are  vseen  to  change  slowly  but  eonstantl}',  the  outline  not 
remaining  the  same  for  any  length  of  time.  This  shifting 
of  form  may  go  on  about  equally  in  all  directions,  the  cor- 
puscles as  a  whole  remaining  stationary  ;  or  there  may  be 
a  flowing  movement  of  the  protoplasmic  body  toward  one 
of  the  lobular  processes,  and  a  consequent  change  of  posi- 
tion,  as    irregidar    and  indefinite  as  the  movements   by 


so  PART  I.     THE  TISSUES. 

which  it  is  caused.  This  mode  of  motion,  character- 
istic of  leucocytes  wherever  found  (and  of  some  other 
elements  as  well)  is  identical  in  its  nature  with  that  seen 
to  take  place  in  the  members  of  the  genus  amoeba,  a 
group  of  very  lowly  organized  unicellular  animals:  it  is, 
herefore,  termed  amoeboid. 

All  the  other  forms  of  colorless  corpuscles  are  probably 
derived  from  the  modification  of  the  small  unicellular  ele- 
ments. These,  in  turn,  are  brought  into  the  blood  stream  by 
the  Wmphatics,  where,  under  the  name  of  lymphocytes, 
they  constitute  the  corpuscular  elements  of  the  h-mph, 
some,  hovk^ever,  attaining  their  full  size  while  in  that  fluid. 

The  origin  of  the  lymphocytes  is  well  known.  They  are 
formed  chiefl}',  if  not  solely,  in  those  organs  which  consist 
in  great  measure  of  w^hat  is  known  as  lymphoid  or  ade- 
noid tissue:  those  known  as  lymphatic  glands,  or,  more 
properly,  as  lymphatic  nodes,  having  the  formation  of 
Ivmphocytes  as  their  principal  if  not  their  sole  function. 
The  histological  anatomv  of  these  organs  will  be  described 
in  a  subsequent  chapter:  it  is  sufficient  here  to  say  that 
they  consist  in  the  main  of  masses  of  adenoid  tissue  well 
supplied  with  blood  vessels  and  enclosed  in  each  instance 
in  a  capsule,  into  which  several  small  Kmphatic  vessels 
enter  and  from  which  a  single  larger  h'mphatic  vessel 
leads,  the  current  of  lymph  which  thus  passes  through  the 
mass,  carrying  with  it  the  newly  formed  Wmph  corpuscles. 

What  is  here  termed  adenoid  tissue  is,  like  the  marrow 
of  bone,  a  compound  structure  of  which  retiform  tissue  is 
the  basis.  In  addition  to  the  flattened  connective  tissue 
corpuscles  which  form  an  endothelioid  layer  upon  the  reti- 


CHAI'TKR    VII.      TMK    BLOOD,  81 

culuin,  ami  which  have  been  described  in  a  previous  chap- 
ter  as   the   fixed  corpuscles  proper  of  relitonn  tissue,  the 
trabeculae  which  make  up  the  framework  of  tlie  juass  are  : -MMri-r»*~4A^ 
1^  in  most  Ccises  so  numerous  as  to  entirely  fill  the  meshes  of  e*r^^Ha,'jLuc 
the  network.     These  cells,  properly  known   as  lympho-  '*^^*' 
blasts,  niultij)ly  rapidly  by  cell  division,  the  older  gradu- 
ally falling  into  the  lymph  channels  which  penetrate  the 
mass  and   becoming  young  lymph  corpuscles  or  lympho- 
cytes.   At  this  stage  they  are  spheroidal,  with  kirge  nuclei 
and   a  very  small   amount  of  investing  protoplasm :  the 
latter  substance  increases  in  quantity  as  they  are  carried 
to  the  heart;   but,  as  we  have  already  seen,  many  enter 
the  blood  stream  before  they  are  fully  matured. 

The  origin  of  the  colored  corpuscles  of  the  blood  is  now 
also  well  established.  During  the  early  stages  of  embry- 
onic life  large  nucleated  colored  corpuscles  are  formed  in 
the  newh^  forming  blood  vessels.  Later,  colored  corpus- 
cles of  the  ordinary  type  ar^ formed  first  in  the  liver,  later 
in  the  spleen,  and  fimilly  in  the  red  niarrow.  After  birth 
tile  last  named  structure  is  the  chief  and  probably  the  sole 
])lace  of  their  formation  in  most  if  not  all  mammals,  at 
least  during  health ;  it  is  possible  that  their  formation 
may  be  temporarily  resumed  by  the  s])leen  if  not  bv  the 
liver  under  certain  abnormal  conditions. 

In  the  description  of  the  marrow  already  given  it  wiis 
stated  that  the  interior  of  that  structure  contains  numer- 
ous small  cells  with  granular  nuclei  known  as  erythl'O- 
blasts :  these  are  formed  by  cell  division  from  larger  cells 
not  distinguishable  from  ordinary'  marrow  cells.  The 
erythroblasts  so  formed  multiply  rapidly  by  the   ordinarv 


82  PART    I.      THE    TISSUES. 

method  of  cell  division,  the  cells  thus  formed  being  sphe- 
roidal and  nucleated  and  gradually  becoming  converted 
into  immature  blood  corpuscles  by  the  formation  within 
them  of  haemoglobin.  Later  the  nucleus  is  extruded  from 
the  corpuscle  and  the  spheroidal  mass  becomes  converted 
into  the  biconcave  disk  which  we  find  in  the  blood  stream. 

Reference  has  been  made  to  the  blood  platelets.  These 
are  minute  rounded  and  colorless  bodies  (from  one-third 
to  one-fourth  the  diameter  of  a  colored  corpuscle)  which 
are  found  in  the  blood  either  singly  or  adhering  together 
in  masses  of  considerable  size:  their  total  number  in  nor- 
mal human  blood  is  but  small.  They  have  been  described 
under  a  number  of  names,  as  the  elementary  particles 
of  Zimmermann,  the  granules  of  Osier,  the  haematc- 
blasts  of  Hayem.  The  name  of  blood  platelets  was  pro- 
posed for  them  b}'  Bizzozero.  They  have  been  regarded 
by  Hayem  and  others  as  concerned  in  the  formation  of  red 
corpuscles  within  the  blood  stream,  but  the  evidence  for 
this  view  is  not  conclusive.  Perhaps  the  most  probable 
explanation  of  them  is  that  they  are  the  disintegrating 
fragments  of  broken  down  colorless  corpuscles.  Our 
knowledge  of  them  is,  however,  exceedingly  imperfect. 


CJIMTKK    VIM.      CoNTUACni.E    TISSUES.  H.'i 


i 


CHAPTER  VIII.  :.«^««».. 


THE  COXTKACTILK  TISSUES, 


What  is  in  physiological  language  termed  contractility 
consists  in  a  change  of  form  but  not  of  volume.  Irregu 
lar  contractility,  or  change  of  form  in  any  direction  and 
in  an  indefinite  manner,  is  one  of  the  powers  which  are 
inherent  in  living  matter  and  may  be  manifested  bv  anv 
cell  which  is  still  in  an  embryonic  condition  :  that  speciali- 
zation of  function  and  structure  which  con  verts  the  embry- 
xm\c  cell  into  the  tissue  dementis  in  many  cases  accom  pan- ?*.£>.  yJ^*^ 
ied  by  the  disappearance  of  this  power:  it  is  retained,  how- 
ever, in  some  cases,  as  in  the  contractility  of  pigment  cor- 
puscles in  many  vertebrates,  and  notably  in  the  amoeboid 
movements  of  leucocytes  which  has  been  described  in  the 
preceding  chapter. 

In  one  group  of  elements,  however,  this  j)ower  is  special- 
ized and  becomes  their  distinctive  function.  This  speciali- 
zation is  of  more  than  one  kind :  there  is  probably  an  in- 
crease in  the  actual  amount  of  contraction,  and  certainlv 
an  increase  in  its  rate;  but  the  most  important  feature  is  its 
detiniteness  of  direction,  one  axis  of  the  mass  (the  long- 
est) always  becoming  shorter,  while  the  mass  as  a  whole 
becomes  thicker.  From  the  fact  that  elements  belonging 
to  this  group  are  the  essentials  of  structure  of  the  organs 
known  as  muscles,  the  tissues  formed  of  them  are  known 


84  .  PART   I.      THE  TISSUES. 

as  muscular  tissues,  although  the  elements  of  which  they 
are  composed  sometimes  occur  singl}^  and  are  frequently 
found  in  masses  that  cannot  with  propriety  be  called  mus- 
cles. 

There  are  three  distinct  kinds  of  muscular  elements  or 
fibres  (as  they  are  commonly  termed,  from  the  elongation 
generalh'  characteristic  of  them)  the  smooth,  the  cardiac, 
and  the  striped  or  striated  muscular  fibres.  The  elements 
of  the  first  two  kinds  have  in  each  instance  a  single  nu- 
cleus, and  may  therefore  be  regarded  as  resulting  from  the 
direct  modification  of  a  single  embrj'-onic  cell.  Those  of 
the  third  kind  are  much  longer  as  well  as  thicker  than  the 
others,  and  while  they  arise  in  each  instance  from  a  single 
embryonic  cell,  this  becomes  greatly  elongated  and  the 
nucleus  divides  repeatedly;  the  resulting  strand  of  proto- 
plasm thus  becoming  multmuclear. 

What  is  variously  called  smooth,  unstriped,  plain  or 
involuntary  muscular  tissue  is  composed  of  spindle 
shaped  cells  or  fibres  whose  protoplasmic  bodies  show  at 
times  quite  distinct  evidences  of  longitudinal  striation, 
but  are  at  other  times  perfectly  plain.  The  existence  of  a 
very  delicate  investing  membrane  or  sheath  has  been  dem- 
onstrated. The  nucleus  is  elongated,  sometimes  oval  but 
in  many  cases  distinctly  rod  shaped,  and  is  situated  in  the 
centre  of  the  mass.  Smooth  muscular  fibres  vary  some- 
what in  size  and  particularly  in  length :  the  transverse 
diameter  usually  ranging  between  five  and  ten  micra, 
while  the  length  maybe  less  than  tenormorc  than  twenty 
times  the  diameter.     When  so  situated  as  to  escape  lateral 


CHAPTER    Vm.      CONTRACTILE   TISSl'ES.  80 

[jress^ure  the  smoolh  fibres  are  cjrculnr  in  cross  section  : 
when,  however,  as  is  frequently  the  case,  thev  are  pressed 
together,  the  sides  become  flattened  and  their  cross  sec- 
tions polygonal.  When  seen  in  transverse  section,  there- 
fore, the  investing  membrane  forms  a  circle  or  polygon, 
within  which  is  seen  the  protoplasm  of  the  body  of  the 
fii^rc,  devoid  of  any  clearly  discernible  structure,  and  in  the 
centre  the  circular  section  of  the  nucleus. 

Smooth  muscular  fibres  are  usually  associated  in  bun- 
dles, the  tapering  extremities  (which  are  sometimes  forked  ) 
overlapping  upon  the  bodies  of  adjacent  fibres  and  adher- 
ing closely  thereto;  a  small  amountof  intercellular  cement 
substance  intervenes,  as  can  be  demonstrated  by  the  use 
of  silver  nitrate.  Little  if  any  skeletal  tissue  pervades  the 
bundles.  The  latter  are  in  some  cases  more  or  less  loosely 
interwoven :  their  most  common  arrangement,  however, 
is  in  more  or  less  extensive  layers;  as,  for  instance,  in  the 
muscular  wall  of  the  intestine:  such  la3'ers  are  penetrated 
by  areolar  tissue  accompanying  the  blood  vessels  and  nerv- 
ous supply  of  the  muscle  fibres.  It  is  very  rarely  the  case 
that  smooth  muscular  fibres  are  aggregated  together  into 
definite  masses  that  can  with  propriety  be  called  muscles, 
their  most  common  occurrence  being  in  the  blood  vessels 
and  the  viscera.  In  no  case  are  they  under  the  control  of 
the  will. 

The  cardiac  fibres  are  found,  as  their  name  implies,  in 
the  muscular  substance  of  the  heart,  both  the  auricles  and 
the  ventricles  beingchiefl}' composed  of  them  :  they  also  con- 
stitute an  important  portion  of  the  walls  of  the  ]ndmon- 


86  PART  I.     THE  TISSUES, 

ary  veins  and  the  superior  and  inferior  venae  cavae  for  a 
short  distance  previous  to  their  openings  into  the  auricles. 
What  are  known  anatomically  as  fibres  are  in  this  case  as 
in  others  aggregates  of  structures  not  visible  to  the  naked 
eve.  In  this  instance,  however,  a  confusion  sometimes 
arises  from  the  application  of  the  term  fibre  to  bodies 
which  are  thus  compared  with  ordinarj'  striped  fibres: 
the}'  are  in  realitj'  rows  of  shorter  elements  more  nearh' 
comparable  to  the  smooth  muscular  fibres. 

The  cardiac  muscular  elements  are  short,  stout,  irreg- 
ularly prismatic  bodies,  intermediate  in  diameter  between 
smooth  and  striped  fibres  and  three  or  four  times  as  long 
as  wnde.  Their  ends  in  some  cases  terminate  squarel}^  in 
others  are  quite  jagged  and  irregular.  They  are  sometimes 
of  uniform  diameter  throughout,  but  many  give  off  short 
branches  which  unite  with  those  from  adjacent  elements, 
Nothing  like  an  investing  membrane  or  sarcolemhia  has 
been  demonstrated.  The  elements  are  faintly  striated 
longitudinall}'  and  more  distinctly  transversely.  Each 
element  has  a  single  nucleus,  surrrounded  b\'  a  compara- 
tively large  amount  of  protoplasm  showing  no  trace  of 
the  structure  which  in  the  superficial  portion  gives  rise  to 
the  appearance  of  striation. 

The  c.ardiac  elements  are  joined  together  by  their  ends 
to  form  the  cardiac  fibres  to  which  reference  has  been 
made,  a  larger  or  smaller  quantity  of  intervening  cement 
substance  being  clearly  demonstrable.  As  many  of  the 
elements  branch  and  anastomose  with  those  of  adjacent 
fibres,  the  appearance  presented  is  that  of  a  network  with 
elongated  and  narrow  meshes. 


CHAPTER    VIII.      CONTRACTILE   TISSUES  S7 

The  various  names  of  striped,  striated,  voluntary  or 
skeletal  muscular  fibres  are  applied  to  those  niultinu- 
eleated  fibres  whieh  form  the  organs  ordinarily  called 
muscles  and  usually  attached  to  the  bony  or  cartilaginous 
skeleton,  the  majority  of  them  being  under  the  control  of 
the  will.  They  are  much  larger  than  the  smooth  or  thecar- 
diac  muscular  elements,  their  transverse  diameter  ranging 
from  ten  to  seventy  micra,  while  they  are  in  some  cases  as 
much  as  three  or  four  centimetres  long:  they  are  prismatic 
in  form,  the  ends  tapering  more  or  less  graduall}-.  Each 
fibre  is  invested  by  a  thin  homogeneous  membrane  known 
as  the  sarcolemma:  within  this  is  the  mass  of  modified 
protoplasm  which  is  the  seat  of  the  function  of  contrac- 
tion :  its  most  conspicuous  feature  is  the  transverse  strip- 
ing or  striation  which  gives  to  the  fibres  the  name  most 
commonlv  applied.  This,  when  seen  b\'  moderate  powers 
of  the  microscope,  presents  to  the  eye  the  appearance  of 
alternating  dim  and  clear  bands;  while  through  the  mid- 
dle of  the  clear  band  may  be  seen  a  narrow  black  line:  a 
longitudinal  striation  may  also  be  seen,  but  usually  less 
distincth'. 

Beneath  the  sarcolemma,  lying  between  it  and  the  con- 
tractile substance,  may  be  seen  here  and  there  elongated 
oval  nuclei:  these  may  be  shown  to  be  surrounded  with  a 
small  amount  of  granular  protoplasm  which  extends  as  a 
thin  disk  for  a  short  distance  around  the  nucleus.  The 
protoplasm  and  the  nucleus  together  make  up  what  is 
known  as  a  muscle  corpuscle:  these,  like  the  similar 
masses  in  the  centre  of  the  cardiac  elements,  may  be  re- 
garded as  the  residuum  after  the  formation  of  the  contrac- 


88  CHAPTER   Vlir.      CONTRACTILE  TISSUES. 

tile  substance.  When  a  cross  section  of  a  bundle  of  striped 
muscular  fibres  is  examined  the  nuclei  are  seen  between  the 
sarcolemmaand  the  contractile  substance,  the  latter  being 
subdivided  into  small  polygonal  areas,  the  fields  or  areas 
of  Cohnheim. 

The  appearances  above  described  are  easily  seen :  the 
explanation  of  the  structure  of  the  contractile  substance 
upon  which  most  of  them  depend  is  still  a  matter  of  dis- 
pute. The  following  facts  are,  however,  quite  generally 
conceded,  and  will  probably  form  the  basis  of  any  further 
positive  addition  to  our  knowledge.  The  contractile  sub- 
stance may  be  regarded  as  made  up  of  a  clear  viscid  or 
semi-fluid  portion,  the  sarcoplasm :  imbedded  in  this  are 
great  numbers  of  elongated  or  rod-like  bodies  (whose  ex- 
act form  is  not  yet  certainly  demonstrated,  and  probably 
varies  with  different  animals) ;  these  are  know^n  as  the 
sarcous  elements.  They  are  quite  uniform  in  length  and 
lie  in  disk-like  groups  w^hich  compose  the  dim  bands  or 
zones  of  the  fibre,  the  clear  zones  being  filled  chiefly  bv 
sarcoplasm  :  the  exact  cause  of  the  dark  line  in  the  middle 
of  the  clear  zone  (known  as  Dobie's  line  or  as  the  mem- 
brane of  Krause)  is  not  yet  certainly  known.  The  sar- 
cous elements  are  not  only  regularly  grouped  across  the 
fibre,  but  also  succeed  each  other  regularly  along  its  length, 
and  are  possibly  united  end  to  end  ;  the  rows  of  sarcous 
elements  constituting  the  fibrillae. 

The  sarcous  elements  are  not  uniformly  distributed 
across  the  fibre,  the  fibrillae  which  thev  compose  being 
grouped  together  in  strands  known  as  muscle  columns: 
These  are  separated  from  each  other  by  sarcoplasm,  and 


CHAPTER    VIII.      CONTK.VCTILK   TISSUES.  89 

the  columns  themselves  arc  somewhat  irregularly  aggre- 
gatcd  in  a  similar  manner.  This  may  best  be  seen  in  the 
cross  section  of  a  fibre,  where  the  arcfis  of  Cohnheim  are 
the  cross  sections  of  the  columns,  the  finely  granular  ap- 
pearance of  their  interiors  representing  the  ends  of  the 
fibrillae;  the  lines  which  bound  the  areas  are  composed  of 
sarcoplasm,  those  which  are  thickest  separating  the  groups 
of  columns  abt)ve  mentioned. 

The  conversion  of  the  whole  of  the  interior  of  the  fibre 
into  sarcoplasm  and  sarcous  elements  and  the  consequent 
lateral  position  of  the  muscle  corpuscles  is  characteristic 
of  most  striped  fibres  of  adult  mammals,  if  not  of  all.  In 
the  young  of  most  mammals,  however,  and  particularly 
in  the  embr\'o,  this  conversion  is  not  entire,  and  the  nuclei 
are  still  found  in  the  interior  of  the  fibre :  a  condition  that 
is  permanent  for  many  of  the  lower  vertebrates.  Such 
fibres  have  also  been  described  in  certain  muscles  of  some 
species  of  mammals  when  fully  grown. 

A  muscle,  in  the  ordinary  sense  of  the  term,  is  an  organ 
consisting  substantially  of  a  mass  of  striped  muscular 
fibre  and  its  associated  skeletal  structures.  It  will  there- 
fore be  convenient  in  this  case,  as  in  those  of  the  cartilages 
and  the  bones,  to  describe  the  histological  anatomy  of  the 
muscles  in  connection  with  their  single  characteristic  tis- 
sue. 

When  seen  with  the  naked  eye  a  muscle  appears  to  be 
made  up  of  readily  distinguishable  fibres  of  varying  fine- 
ness :  these,  which  are  the  anatomical  fibres,  are  bundles 
or  fasciculi  of  the  elements  which  are  termed  fibres  in  the 


90  PART  I.      THE  TISSUES. 

histological  sense.  Each  fasciculus  is  invested  b\^  a  layer 
of  areolar  tissue  continuous  upon  its  outer  surface  with 
that  of  those  adjacent  and  giving  off  from  its  inner  surface 
delicate  septa  which  lie  between  the  individual  fibres.  The 
investing  layer  of  the  fasciculus  is  known  as  the  perimys- 
ium; the  internal  skeletal  tissue  as  the  endomysium. 
The  aggregated  fasciculi  which  make  up  the  body  of  the 
muscle  are  invested  as  a  whole  by  a  layer  of  connective 
tissue  continuous  with  the  outer  perimysial  layers,  termed 
the  epimysium.  The  arteries  and  veins  proper  to  the  mus- 
cle are  chiefly  located  in  the  perimysium,  while  the  capil- 
lary network,  whose  meshes  are  as  a  rule  greatly  elon- 
gated in  the  direction  of  the  fasculi,  are  situated  in  the  en- 
domysium in  such  a  manner  as  to  be  in  close  proximity  to 
every  fibre. 

The  striped  muscular  fibres  terminate  by  rounded  or 
obliquely  truncated  ends,  which  are  closely  applied  to  the 
correspondingly  shaped  extremities  of  white  fibre  bundles, 
the  sarcolemma  of  the  muscular  fibre  being  directly  contin- 
uous therewith :  these  bundles  are  in  some  cases  almost 
immediately  connected  with  the  periosteum  of  a  bone  or 
some  similar  place  of  attachment:  in  other  instances  they 
are  prolonged  beyond  the  muscle  in  a  fibrous  mass,  the 
tendon  of  origin  or  of  insertion,  as  the  case  may  be. 


Cn.VrTKK    IX.      SMALL    VKSSKLS.  91 


chapti:r  IX. 

TUB  SMALL  VLSSlvLS. 


Mention  was  made  in  the  introductorv  ehapter  of  the 
iaet  that  eertain  tissue  aggregates,  while  themselves  de- 
serving to  rank  as  organs,  sustain  the  same  relation  to 
larger  and  more  complex  organs  as  do  the  tissues  them- 
selves. Among  these  compound  factors  of  structure,  as 
they  were  there  termed,  the  most  important  are  the  small 
blood  and  lymph  vessels,  particularly  the  former.  As  they 
are  built  up  of  endothelial,  skeletal  and  muscular  tissues, 
their  structure  may  now  properly  be  described.  The  fol- 
lowing statements  apply,  however,  only  to  those  smaller 
vessels  which  enter  the  structure  of  other  organs.  The 
larger  vascular  trunks  will  be  described  in  a  subsequent 
chapter  in  connection  with  the  other  organs  of  the  circu- 
latory system. 

The  blood  vessels  are  commonly  distinguished  as  arter- 
ies, which  carry  the  blood  from  the  heart,  veins,  which 
return  it  to  the  heart,  and  capillaries,  which  intervene 
between  the  arteries  and  the  veins,  and  in  which  the  blood 
is  brought  into  the  closest  proximit}'^  to  the  tissues  possi- 
ble in  a  closed  system  of  vessels.  The  lymph  vessels  origi- 
nate in  the  interstitial  spaces  of  the  tissues  (chiefly  if  not 
solely  the  connective  tissues),  these  communicating  directly 
with   the  open   mouths   of  very    small    and    thin-walled 


92  PART    I.      THE    TISSUES. 

vessels  known  as  the  lymph  capillaries:  these  unite  to 
to  form  the  larger  vessels  sometimes  called  lymph  veins 
since  they  convey  their  contents  toward  the  heart,  but 
more  commonly  spoken  of  as  lymphatics. 

The  interior  of  a  small  artery,  such,  for  example,  as  can 
be  just  distinguished  with  the  unaided  eye,  is  lined  with  a 
laj^er  of  endothelium,  whose  cells  are  as  a  rule  greatly 
elongated  in  the  direction  of  the  vessel ;  the  nuclei  also 
being  elongated.  Beneath  this  is  a  layer  of  elastic  tissue 
usually  in  the  form  of  a  membrane,  either  hotnogeneous 
or  fenestrated,  but  occasionally  composed  of  reticulated 
fibres.  The  endothelium  and  the  elastic  layer  make  up 
what  is  usually  called  the  intima,  or  inner  coat ;  the  term 
is,  however,  applied  by  some  histologists  to  the  elastic 
layer,  to  the  exclusion  of  the  endothelium. 

Beneath  the  intima  is  the  middle  coat,  or  media;  this, 
in  the  vessels  under  consideration,  consists  almostentirelv 
of  smooth  muscular  fibres,  arranged  in  a  la^^er  several  cells 
deep,  the  long  axes  of  the  fibres  crossing  the  vessel  at 
right  angles  or  nearly  so.  Like  all  layers  of  muscular  tis- 
sue, this  is  highly  elastic;  during  life  it  is  always  upon  the 
stretch  ;  and  the  contraction  which  usually  takes  place  in 
it  after  death  throws  the  intima  into  longitudinal  folds, 
which,  when  seen  in  cross  section,  give  an  undulating  out- 
line to  the  interior  of  the  arter}^  which  is  highly  charac- 
teristic. 

External  to  the  media  is  the  adventitia,  or  outer  coat : 
this  consists  in  the  smaller  arteries  of  a  la3'er  of  connective 
tissue  in  most  cases  clearly  definable  on  the  one  hand  as 


CIIAPTEK    IX.      SMALL    VESSELS.  93 

pertaining  to  the  artery,  on  the  other  passing  over  more 
or  less  gradually  into  the  adjacent  interstitial  tissue. 
/     As  the  small  arteries  divide  and  subdivide,  finally  becom- 
ing lost   in   the  ca|)illaries,  there  is   a  gradual  reduction 
alike  of  the  adventitia,  the  media  and  the  intima.    The  for- 
mer becomes  reduced  to  a  layer  of  extreme  thinness :  the 
media   diminishes   until  it  is  reduced  to  a  single  layer  of  j 
/transverse  smooth  muscular  fibres  and  later  to  scattered  V 
^fibres  not  in  contact  and  not  forming  a  continuous  layer:  i 
the  elastic  layer  of  the  intima  is  similarh'  reduced  in  ex- 
tent, and  finally  disappears;  the  last  and  least  of  the  ves- 
sels that  may  with  propriety  be  called  arterial  consisting 
merely  of  the  endothelial  lining  and  an  imperfectly  contin- 
iuous  layer  of  smooth  muscular  fibres,  surrounded  more  or 
less  definitely  by  a  few  branched  connective  tissue  corpus-"' 
les. 

The  capillaries  are  the  direct  continuation  of  the  arter- 
ies, arising  either  by  the  farther  subdivision  of  the  struct- 
ures just  described  or  springing  directK%  as  in  some  cases, 
from  the  sides  of  vessels  still  distinctly  arterial  in  their 
structure.  In  either  case  they  branch  freely,  forming  a  net- 
work, whose  meshes  have  a  size,  form  and  direction  in 
direct  relation  with  the  structure  of  the  organ  in  which 
they  occur.  Thc}^  do  not,  like  the  arteries,  become  smaller 
as  they  branch,  those  of  any  one  network  being  approxi- 
mately of  the  same  size,  though  they  may  vary  consider- 
ably in  different  parts  of  the  same  organ.  Their  size  in 
life  is  not  easily  determined,  but  the  majority  of  them  are 
probably  not  over  ten  micra  in  diameter,  though  in  some 


94  PART    I,     THE    TISSUES. 

tissues,  notably  in  the  marrow,  they  may  be  as  much  as 
twice  as  large. 

"  In  structure  the  capillaries  are  simple  tubes  (usually 
cylindricEil  in  form)  of  elongated  endothelial  cells  of  such 
width  that  from  two  to  four  may  be  seen  in  the  cross  sec- 
tion of  a  single  capillary.  As  in  other  endothelial  layers, 
the  cells  are  united  by  an  intercellular  cement  substance: 
here  and  there  patches  of  this  substance  may  be  demon- 
strated by  the  silver  method  which  may  be  regarded  as 
indicating  gaps  between  the  cells  ;  these  have  been  termed 
stigmata;  and  it  is  probable  that  such  places  offer  favor- 
able opportunity  for  that  migration  of  the  leucocytes 
from  the  blood-stream  into  the  tissues  which  is  known  to 
be  normal  to  them.  Capillaries  are  almost  always  situ- 
ated in  interstitial  areolar  tissue  or  its  equivalent,  and  are 
in  direct  relation  with  the  connective  tissue  corpuscles :  in 
some  organs  branched  corpuscles  appear  to  form  an  almost 
continuous  layer  investing  the  capillaries :  such  a  layer  has 
been  termed  an  adventitia  capiilaris. 

As  the  capillaries  originate  by  the  subdivision  of  the 
arteries,  so  their  union  forms  the  origin  of  the  veins.  A 
small  vein  resembles  a  small  artery  in  its  general  struct- 
ure, its  wall,  like  that  of  the  latter,  being  distinguishable 
into  an  intima,  a  media,  and  an  adventitia  :  the  chief  differ- 
ences between  them  may  be  stated  briefly  as  follows.  The 
endothelium  of  the  veins  is,  as  a  rule,  composed  of  shorter 
and  broader  cells  than  that  of  the  arteries,  and  the  elas- 
tic layer  of  the  intima  is  thinner:  the  media  is  very  much 
thinner,  the  amount  of  muscular  tissue  being  very  much 


CIIAPTKK    IX.      SMALL    VESSELS.  95 

reduced:  the  advcntiLia,  which  is  the  principal  coat  of  tlic 
veins,  is,  if  anythiiij^,  thicker  than  that  of  the  correspond- 
ing artery,  and  is  composed  largely  of  fibrous  tissue.  The 
veins  are  as  a  rule  of  a  greater  diameter  than  the  arter- 
ies which  they  accompany. 

Unlike  tlic  arteries,  the  veins  arc  not  during  life  continu- 
allv  on  the  stretch;  and  the  recoil  mentioned  in  connection 
with  the  former  vessels  does  not  take  place  after  death  : 
this  reaction  on  the  part  of  the  arterial  wall  tends  to 
drive  the  contained  blood  through  the  capillaries  and  into 
the  veins:  as  a  rule,  therefore,  the  arteries  are  empty  after 
death,  but  retain  their  patency  on  account  of  the  thickness 
and  elasticity  of  their  walls.  The  veins,  on  the  other  hand, 
are  usually  filled  with  blood;  or  if  empty  become  col- 
lapsed and  flattened  on  account  of  the  thinness  and  inelas- 
ticity of  their  walls. 

A  small  artery  and  a  small  vein  may  therefore  usually 
be  distinguished  when  seen  in  cross  section  by  the  follow- 
ing differences  :  The  artery  is  cjrcular  inform  :  it  is  in  most 
cases  empty,  or  contains  but  a  small  amount  of  blood 
dot:  the  intima  is  thrown^  into  folds,  giving  the  elastic 
layer  a  sinuous  contour,  upon  whicli  the  nuclei  of  the  endo- 
thelial cells  are  often  peculiarly  conspicuous :  the  media  is 
quite  thick,  consisting  chiefly  of  numerous  muscular  fibres  : 
the  adventitia  is  comparatively  weak,  and  the  least  con- 
spicuous of  the  three  coats.  The  vein  may  be  either  circular 
or  more  or  less  irreg^ular  in  forni :  if  thejormer,  it  is  usually 
filled  with  blood  clot,  readilv  recognizable  from  the  numer- 
ous colored  corpuscles,  devoid  of  nuclei  and  appearing  as 
clear  circles,  and  the  few  leucocytes  with  distinct  nuclei: 


96  PART  I.     THE  TISSUES. 

the  intima  is  thin  and  siraj)le_injc:ontour :  the  media  is  also 
thin,  containing  but  few  smooth  muscular  fibres :  the  ad- 
ventitia  is  often  the  thickest  and  most  conspicuous^  the 
three  coats.  Where  the  artery  and  vein  He  side_bj5?_side, 
the  latter  is  usuall}^  the  larger. 

A  similar  application  of  the  principles  of  structure  above 
described  w^ill  enable  the  student  to  interpret  the  different 
appearances  seen  when  a  vessel  is  cut  obliquely  or  longi- 
tudinally, and  with  practice  to  recognize  an  artery  or  a 
vein  wherever  met  with  in  a  section  passing  through  one 
of  the  organs  of  the  body. 

The  interstitial  spaces  of  the  tissues  form  in  many  cases 
an  irregular  network,  in  others  definite  lacunae  or  chan- 
nels for  the  lymph,  a  rather  indefinite  terra  applied  to  the 
fluid  originally  derived  from  the  blood  and  destined  to  be 
returned  to  it  again  by  way  of  the  lyraphatic  vessels.  The 
sraallest  of  these,  the  lymph  capillaries,  arise  from  the 
lymph-spaces  of  the  tissues  in  an  exceedingly  irregular 
manner,  the  connective  tissue  corpuscles  in  many  cases  pass- 
ing by  an  insensible  transition  into  their  endothelium. 
When  fully  formed  they  consist  of  tubes  which  resemble 
blood  capillaries  in  consisting  solely  of  endothelial  cells, 
but  differ  from  those  structures  in  the  fact  that  they  are 
rarely  if  ever  cylindrical,  being  exceedingly  irregular  in 
form,  though  usually  flattened,  and  very  often  quite  vari' 
able  in  diameter:  they  anastomose  freely,  forming  net- 
works of  very  irregular  meshes.  The  cells  of  which  they 
are  composed  are  usually  about  as  long  as  broad,  but  are 
usually  characterized  by  a  distinct  sinuosity  of  outline. 


CMAPTKK    IX.      SMALL   VESSELS.  97 

The  Iviiiph  c.'ipillaries  unite  to  I'nnu  the  lar;.,a'r  vessels 
known  as  llie  lymphatics.  These  do  not  differ  materially 
from  the  former  exee|)t  in  size  and  in  the  presence  of  valvu- 
lar folds.  Like  the  capillaries,  they  consist  almost  entirely 
of  endothelial  cells,  which  are,  however,  more  elongated  in 
the  direction  of  the  vessel  than  those  forming  the  capilla- 
ries, but  having  the  same  sinuous  outline.  It  should  be 
stated  here  that  bv  some  histologists  it  is  held  that  the 
characteristic  irregular  outlines  seen  in  preparations  of 
the  lymphatic  endothelium  by  the  silver  method  are  due 
solely  to  shrinkage  of  the  tissues  after  death,  the  outlines 
of  the  cells  in  the  living  tissues  being  far  simpler. 

A  lymphatic  when  seen  in  cross  section  in  the  interior  of 
an  organ  appears  as  an  opening  of  irregular  form,  its  sides 
usually  approaching  each  other  (due  to  the  flatness  of  the 
vessel) ;  it  can  in  many  cases  hardly  be  distinguished  from 
a  simple  tear  or  fissure  in  the  connective  tissue,  save  by 
the  definiteness  of  its  outline:  this,  except  in  cases  where 
the  vessel  is  quite  collapsed,  is  made  up  of  simple  curves; 
and  in  it  may  occasionally  be  seen  the  nuclei  of  the  endothe- 
lial cells  of  which  it  is  composed.  In  some  cases  the  open- 
ingis  larger  and  more  nearly  equal  in  its  various  diameters, 
resembling  the  cross  section  of  a  vein  :  it  can  then  be  dis- 
tinguished from  the  latter  b^-^the  greater  thinness  and  sim- 
plicity of  its  walls  and  by  the  fact  that  any  clot  which  ma}' 
be  present  consists  almost  entirely  of  fibrin,  appearing 
granular  in  the  section,  with  here  and  there  a  few  lymph- 
ocytes with  large  nuclei  and  a  small  amount  of  surround- 
ing protoplasm. 


98  PART  I.      THE  TISSUES. 

The  structure  of  the  larger  lymphatic  vessels  v/ill  be  de- 
scribed iti  connection  with  the  circulatory  system. 

The  lymphatics  have  a  distribution  closely  related  to 
that  of  the  blood  vessels :  and  it  is  not  unusual  to  see  a 
small  artery,  its  companion  vein,  and  one  or  more  lym- 
phatics in  close  proximity.  In  some  cases  a  blood  vessel 
may  be  situated  within  a  lymphatic  vessel.  Such  lym- 
phatics are  termed  perivascular. 

The  serous  cavities  of  the  body  are  in  reality  enlarged 
lymph  cavities.  They  are  lined  by  the  serous  membranes, 
which  are  known  by  distinctive  names  (e.  g.,  pleura),  ac- 
cording to  their  position.  These  consist  in  each  case  of 
connjective  tissue  which  contains  a  more  or  less  well  de- 
veloped network  of  fine  elastic  fibres,  surmounted  by  a 
homogeneous  basement  membrane.  On  this  rests  the 
serous  endothelium,  a  layer  of  polygonal  pavement  cells 
of  varying  size.  Here  and  there  ma^-  be  found  small  open- 
ings called  stomata,  which  put  the  serous  cavities  in  com- 
munication with  the  lymph  channels  of  the  membrane  and 
thus  with  the  lymphatic  system  :  they  are  surrounded  by 
cells  which  are  usually  more  tumid  and  granular  than 
those  adjacent. 

In  addition  to  the  tumid  cells  which  surround  the  sto- 
mata there  may  sometimes  be  found  on  the  serous  surfaces 
patches  of  granular  cells,  which  may  be  cuboidal  if  not 
columnar  in  form:  these  give  evidence  of  rapid  cell  divi- 
sion, and  may  be  regarded  as  local  centres  for  the  forma- 
tion of  the  leucocytes  which  may  be  found  in  the  serous 
cavities. 


A^  ha^jik^ 


CHAI'TFR   X.      NERVOUS  TISSUES.  99 


CHAPTER  X. 

THE  NERVOUS  TISSUES. 


The  nervous  tissues  have  for  their  characteristic  function 
the  reception,  conduction,  distribution,  and  discharge  of 
impulses  or  stimuli.  The  stimuli  received  may  be  such  as, 
when  manifested  in  consciousness,  we  know  as  sensations  ; 
or  the\'  may  be  such  as  are  never  reported  to  our  con- 
sciousness :  in  either  case  they  may  be  either  rnechanical, 
physical,  or  chemical  in  their  origin:  the  distance  to  which 
they  are  transmitted  may  vary  greatl}^  in  different  cases, 
as  may  also  the  extent  and  nature  of  their  distribution. 
The  impulses  which  are  discharged  (with  or  without  pre- 
vious distribution)  must  be  regarded  as  liberations  of 
energy  due  to  changes  set  up  in  the  central  nervous  ele- 
ments by  the  ingoing  stimuli :  the\'  ma^'  or  may  not  re- 
sult in  associated  and  more  conspicuous  liberations  of 
energy  in  the  muscular  tissues. 

The  nervous  elements,  are,  then,  like  the  muscular  ele- 
ments, reservoirs  of  energy :  they  differ  from  them  both 
functionally  and  structurally  by  what  has  been  termed 
their  polarity,  by  which  is  expressed  the  fact  that  the  im- 
pulses received  enter  in  a  more  or  less  definite  direction, 
while  the  ensuing  discharge  takes  place  along  a  definite 
line  which  is  in  a  general  way  in  an  opposite  direction  to 
that  along  which  the  incoming  stimulus  enters.  A  nervous 


100  PART  I.      THE  TISSUES. 

element  of  the  simplest  type  may  be  conceived  of  as  com- 
prising a  spheroidal  or  spindle-shap^ed  rnass  of  protoplasm 
provided  with  a  nucleus,  each  of  whose  extremities  is  con- 
tinued into  a  filament  of  more  or  less  length.  Along  one 
of  these  filaments  stimiuli  travel  toward  the  central  corpus- 
cle, and  the  extremity  of  the  filament  is  modified  for  their 
reception:  along  the  other,  nervous  impulses  pass  outward 
to  their  appropriate  place  of  discharge,  its  extremity  show- 
ing corresponding  modification.  The  whole  structure, 
from  one  set  of  terminals  to  the  other  (and  including  both) 
is  the  result  of  the  modification  of  a  single  embryonic  cell : 
on  account  of  the  delicacy  of  the  various  parts,  however, 
and  the  great  length  in  many  cases  of  the  conducting  fila- 
ment, it  is  rarely  (if  ever)  possible  to  isolate  such  a  nervous 
element  in  its  entirety,  as  we  can  isolate  a  smooth  muscu- 
lar fibre  or  an  epithelial  cell:  nor  can  nervous  elements 
(with  a  few  exceptions)  be  so  prepared  that  the\^  can  be 
seen  in  their  entirety  in  a  single  section :  in  most  cases  our 
study  must  at  any  one  time  be  chiefly  directed  to  one  or 
the  other  of  the  regions  above  indicated.  Practicallv,  it 
has  been  customary  to  distinguish,  as  the  structural  fac- 
tors of  the  nervous  system,  the  conducting  strands  of 
greater  or  less  length  along  which  impulses  are  transmit- 
ted ;  the  central  corpuscular  masses ;  and  the  terminals  of 
reception  and  discharge:  and  although  these  are  now  known 
not  to  be  independent  parts,  it  is  convenient  to  retain 
this  distinction  as  a  basis  for  their  description:  according, 
therefore,  to  the  form  and  associated  function  of  the  vari- 
ous components  of  the  nervous  system,  it  is  customary 
to  classify  them  as  follows. 


CHArTKK    X.      NKRVors   TISSUKS.  101 

A.  ComlucLiiiL,^  elements,  or  nerve  fibres :  ihese  consist 
in  every  case  of  continuonsev  linilrical  ( or  rounded)  strands 
w  hieh  run  without  interruption  Ironi  the  central  cor|3us- 
cles  to  the  terminal  modifications  ol  their  free  extremities. 
The  presence  or  absence  of  certain  investing  structures  dis- 
tinguishes two  kinds  of  nerve  fibres,  known  as 

1.  Non-medullated  fibres:  also  called  gray  or  gel- 
atinous fibres,  or  (from  their  discoverer)  the  fibres 
of  Remak:  the  axis  consists  of  a  grayish  cylinder 
or  a  band  with  rounded  edges,  showing  delicate 
longitudinal  striations  :  they  are  invested,  if  at  all, 
by  a  very  delicate  primitive  sheath  whose  exist- 
ence is  questioned  by  many  histologists.  Upon  the 
surface  of  the  fibre  are  seen  numerous  nuclei.  The 
gray  fibres  not  unfrequently  branch  and  anasto- 
mose with  adjacent  fibres.  ^ 

2.  Medullated  fibres:  also  called  white  fibres:  the  O^^^ 
•axis-cylinder  is  grayish  and  longitudinally  striat- 
ed :  it  is  surrounded  by  a  layer  of  a  fatty  substance 

termed  myelin,  the  layer  being  called  the  medul- 
lary sheath  or  white  substance  of  Schwann: 
outside  of  this  is  the  clearl}^  defined  primitive 
sheath  or  neurilemma.  The  medullary  sheath  is 
interrupted  at  frequent  intervals,  the  points  where 
this  occurs  being  termed  the  nodes  of  Ranvier ; 
the  intervening  segments  are  called  internodes: 
each  exhibits  one  or  two  so-called  nerve-fibre  cor- 
puscles situated  between  the  medullary  sheath  and 
the  neurilemma  and  consisting  of  oval  nuclei  spar- 
ingly invested  with  protoplasm.  Medullated  fibres 
rarely  branch,  except  near  their  extremities. 


102  PART  I.      THE  TISSUES. 

B.  Central  elements,  or  nerve  corpuscles:  these  are 
variously  shaped  bodies  with  conspicuous  nuclei:  they 
vary  greatly  in  size  also,  some  being  among  the  smallest 
and  others  among  the  largest  of  the  tissue  elements.  Their 
surfaces  always  give  off  one  or  more  processes  termed 
poles:  according  to  the  number  of  these  processes  they 
are  commonly  distinguished  as  unipolar,  bipolar,  and 
multipolar  corpuscles.  Most  nerve  corpuscles  (in  the 
higher  vertebrates,  at  least)  are  multipolar;  and  in  the 
great  majority  of  instances  one  of  the  poles  of  a  multipol- 
ar corpuscle  can  be  distinguished  from  the  x)thers  as  the 
process  of  Deiters,  or,  better,  as  its  axis-cylinder  process : 
the  rest  are  then  known  as  protoplasmic  processes  and 
rapidly  divide  into  irregular  branches  termed  dendrites : 
in  some  multipolar  corpuscles  no  axis-cylinder  process  can 
be  distinguished ;  such  corpuscles  are  termed  amacrine. 
Other  features  will  be  discussed  in  a  subsequent  para- 
graph. 

C.  Peripheral  elements,  or  nerve  terminals,  formed  by 
the  modification  of  the  extremities  of  nerve  fibres  with  or 
without  epithelial  or  skeletal  elements.  According  as  they 
are  situated  at  the  extremities  of  fibres  which  conduct  im- 
pulses toward  the  nerve  centres  (afferent  fibres)  or  away 
from  them  (efferent  fibres)  they  are  distinguished  as 

1.  Receiving  terminals:  of  these  various  kinds  are 
known,  composing  two  groups  which  differ  from 
each  other  alike  in  the  form  and  in  the  arrangement 
of  the  structures  included.  In  one  group  large  num- 
bers of  similar  terminals  are  associated  with  modi- 
fied epithelial  cells  to  form  the  essential  structures 
of  the  three  great  organs  of  special  sense,  the  nose. 


f) 


CHAPTER    X.      NKKVnCS  TISSUI-S.  103 

Fcri|)luM'al  cleinenls  or  nerve  terminals  (continued). 
the  eye,  and  the  ear:  these  will  be  described  in  a 
subsequent  chapter,  together  with  the  less  highly 
sjjecialized  organs  of  tiiste.  In  the  other  the  term- 
inals are  always  either  solitary  or  in  groups  of  two 
or  three,  though  they  mav  be  more  or  less  al)und- 
ant.  Some  of  these  forms  of  terminals  have  been 
regarded  as  associated  with  specific  modes  of  sen- 
sation, but  the  function  of  most  of  them  is  alto- 
gether unknown :  they  are  therefore  usually  dis- 
tinguished by  peculiarities  of  form  or  of  location, 
or  by  the  names  of  their  discoverers.  The  principal 
forms  ma}'  conveniently  be  described  here. 

a.  The  simplest  form  of  receiving  terminal  is  seen 
in  what  are  known  as  free  endings.  In  these  the 
nerve  fibre  first  looses  the  investing  white  sub- 
stance of  Schwann,  and  later  the  neurilemma, 
though  the  nerve-fibre  corpuscles  are  still  seen  for 
a  while  before  the  axis-cylinder  becomes  entireh- 
naked.  When  the  latter  stage  is  reached,  the  fibrils 
are  rapidly  split  up  into  small  bundles,  and  finally 
form  a  tuft  or  pencil  of  delicate  more  or  less  vari- 
cose filaments  which  ramify  among  the  cells  of  the 
epithelium  in  the  case  of  the  skin,  a  mucous  mem- 
brane, or  the  substance  of  a  gland  ;  or  among  the 
elements  of  other  organs.  Branching  terminals, 
essentially  similar,  found  in  tendons,  are  known  as 
organs  of  Golgi. 

b.  Closely  allied  to  the  preceding  are  the  terminals 
associated  with  what  are  known  as  tactile  cells : 
these  are  spheroidal  cells  found  in  the  deeper  por- 
tion of  the  epidermis  and  probabl}'  epithelial  in 
character.  The  nerve  fibre  in  relation  with  a  group 


104  PART   I.      THE  TISSUES. 

C.  Peripheral  elements  or  nerve  terminals  [continued). 
of  them  looses  its  investments  and  breaks  up  into 
a  number  of  slender  branches,  each  of  which  ends 
in  a  saucer-shaped  disk,  the  tactile  meniscus, 
which  embraces  the  proximal  surface  of  the  tactile 
cell,  so  called.  There  is  no  positive  evidence  that 
these,  rather  than  other  cutaneous  terminals,  are 
the  special  organs  of  touch. 

C.  The  bodies  known  as  compound  tactile  cells, 
also  sometimes  called  the  corpuscles  of  Grandry, 
found  in  the  skin  of  certain  birds,  and  also  de- 
scribed as  occurring  in  mammals,  may  be  regarded 
as  composed  of  two  or  more  layers  of  tactile  cells, 
and  having  between  adjacent  layers  terminal  ex- 
pansions known  as  tactile  disks.  The  neurilemma 
of  the  nerve  fibre  of  which  the  tactile  disks  are  the 
termination  becomes  continuous  with  a  connective 
tissue  capsule  that  invests  the  whole  structure. 

d.  The  name  of  tactile  corpuscles  has  long  been 
given  to  large  ovoid  bodies  found  in  the  papillae  of 
the  skin  of  the  hand  and  foot  and  in  other  places 
whore  the  sense  of  touch  is  well  developed  :  they  are 
also  called  the  corpuscles  of  Meissner.  Each  is  com  - 
posed  of  a  mass  of  connective  tissue  about  which  a 
medullated  fibre  winds  spirally  once  or  twice,  the 
sheaths  of  the  fibre  then  becoming  merged  in  the 
mass  or  continuous  with  its  capsule:  the  axis- 
C3dinder  passes  as  a  nonmedullated  fibre  into  the 
interior,  where  it  branches  more  or  less  freely,  the 
branches  becoming  varicose.  Two,  three,  or  even 
four  medullated  fibres  may  be  connected  with  a 
single  large  corpuscle  of  Meissner. 

e.  What  are  termed  end-bulbs  are  spheroidal  or 
cylindroidal  bodies  of  simple  structure  in  which  the 
axis  cylinder  of  a  medullated  fibre  enters  the  proxi- 


CIIAPTKR    X.      NICKVOUS   TISSUES.  105 

C.  Peripheral  elements  or  nerve  terminals  (continued). 
mal  end  of  a  mass  of  connectivctissue  which  is  con- 
tinuous with  the  sheaths  of  the  fibre,  and  extends 
throughout  its  length,  branching  but  little  if  at  all. 
They  are  found  in  the  conjunctiva  of  the  eye,  and 
in  other  modified  dermal  structures,  as  well  as  in- 
ternally. The  articular  corpuscles  found  in  the 
vicinity  of  joints,  as  well  as  the  genital  corpuscles 
of  the  male  and  the  female  sexual  organs  are  prob- 
ably to  be  regarded  as  modified  end-bulbs,  though 
both  are  held  by  some  histologis  ts  to  approach  more 
nearly  in  structure  to  the  corpuscles  of  Meissner. 

f.  The  Pacinian  bodies,  or,  as  thev  are  sometimes 
called,  the  corpuscles  of  Vater,  are  the  largest  of 
the  terminals,  being  easily  visible  to  the  naked  eye 
in  man\'^  cases.  They  are  irregularly^ ovoid  in  form, 
each  having  for  its  axis  the  axis-cylinder  of  a  med- 
ullated  nerve-fibre,  which  may  terminate  near  the 
extremity  of  the  corpuscle  by  a  bulbous  enlarge- 
ment or  may  divide  near  the  end  into  short  irregu- 
lar branches  wnth  pyriform  extremities.  The  axial 
structure  is  imbedded  in  a  cylindrical  core  of  doubt- 
ful nature:  it  is  faintly  granular  and  contains  scat- 
tered nuclei,  and  is  possibly  homologous  with  the 
principal  mass  of  an  end-bulb.  Surrounding  this 
is  a  series  of  concentric  tunics  which  maybefiftyor 
more  in  number,  each  consisting  of  a  fibrous  layer 
and  an  endothelial  investment:  the  whole  may  be 
regarded  as  a  highh^  specialized  modification  of 
the  capsule  of  the  end-bulb.  Pacinian  bodies  are 
found  widely  distributed  throughout  the  bodv.  As 
is  the  case  with  the  other  forms  of  receiving  termi- 
nals, we  have  as  yet  no  certain  knowledge  of  their 
specific  function. 


lOG  PART   I.      THE  TISSUES. 

C.  Peripheral  elements  or  nerve  terminals  {continued). 
2.  Discharging  terminals :  Impulses  sent  out  from 
the  nerve  centres  may  give  rise  to  muscular,  glan- 
dular, or  other  activities.  The  terminals  by  which 
discharge  is  made  upon  the  elements  of  the  tissues 
involved  have  in  every  case,  as  far  as  known,  the 
form  of  ramifications  or  arborizations  of  the  ex- 
tremities of  efferent  fibres.  In  the  case  of  glands 
or  other  secretorv  structures  the  terminal  subdivi- 
sions are  situated  among  the  epithelial  cells.  In 
the  case  of  smooth  muscular  tissue,  nonmedullated 
fibres  from  an  adjacent  ganglion  or  ganglionic 
plexus  enter  the  muscular  layer,  between  whose 
elements  the  ramifications  of  the  fibre  are  situated. 
The  mechanism  of  discharge  in  the  case  of  striped 
muscular  tissue  is  somewhat  more  complex.  Me- 
dullated  fibres  from  the  intramuscular  plexus,  fol- 
lowing the  endomysium,  divide  in  each  instance 
into  two  or  more  branches,  each  branch  passing 
to  a  single  muscular  fibre.  The  medullary  sheath 
disappears,  the  neurilemma  apparentU^  becomes 
continuous  with  the  sarcolemma  of  the  muscular 
fibre,  and  the  axis-cylinder  breaks  up  into  a 
number  of  fine  varicose  branches :  the  latter  rest 
upon  or  are  imbedded  in  the  sole-plate,  a  flat- 
tened granular  mass  of  protoplasm  containing 
several  nuclei  and  lying  between  the  sarcolemma 
and  the  body  of  the  fibre:  the  whole  structure  is 
termed  a  motor  end-plate.  By  some  histologists 
the  end-plates  are  believed  to  be  situated  altogether 
outside  of  the  sarcolemma. 


CHAPTKR    X.      NERVOI'S   Tl«Sl'ES.  1(>7 

Attention  has  l)ccn  called  to  the  delicate  longitudinal 
striation  of  the  axis  cvlinder  of  the  medullated  fibre  and 
of  the  corresponding  portion  of  the  gray  fibre.  This  is 
the  expression  of  a  distinct  fibrillation,  the  primitive 
fibrillae  l)eing  imbedded  in  an  intervening  homogeneous 
substance,  the  neuroplasm  (the  resemblance  to  the  fibril- 
lae and  sarcoplasm  of  striated  muscular  fibre  is  note- 
worthy). The  terminal  subdivision  common  to  medul- 
lated and  nonmedullated  fibres  consists  of  a  breaking  up 
of  the  axis  into  smaller  bundles  of  these  fibrillae  and  even- 
tually, in  some  cases,  to  the  separation  of  each  individual 
fibrilla. 

The  gray  or  nonmedullated  fibres  consist  of  little  more 
than  bundles  of  fibrils  and  neuroplasm.  Those  which 
compose  the  branches  of  the  olfactory  nerves  have  a  well- 
defined  and  nucleated  primitive  sheath:  in  most  gray 
fibres  no  such  sheath  can  be  demonstrated.  Scattered 
nuclei  are  seen  upon  the  surface  of  the  gray  fibres:  these, 
like  the  sheath,  when  present  must  be  regarded  as  skeletal 
rather  than  nervous  in  character.  Gray  fibres  show  well 
marked  varicosities,  w'hich  are  possibU'  due  to  local  accu- 
mulations of  neuroplasm,  the  fibrillae  being  correspond- 
ingly separated  at  such  points.  Where  gray  fibres  branch 
and  anastomose  bundles  of  fibrillae  accompanied  by  neu- 
roplasm pass  over  from  one  fibre  to  another:  the  angles 
formed  by  the  branches  are  frequently  filled  for  a  short 
distance  with  neuroplasm,  and  investing  nuclei  are  often 
relatively  abundant  at  such  points. 

The  axis  cylinders  of  white  fibres  show  under  certain 
methods  of  treatment  transverse  striation  curiouslv  like 


108  PART   I.      THE   TISSUES. 

that  of  a  striped  muscular  fibre:  if  a  reality,  and  not  due 
merely  to  the  reagents  used  for  its  demonstration,  this  must 
be  catised  b\'  regular  variations  in  the  size  of  the  fibril- 
lae.  The  axis  cylinder  may  also  be  shown  to  be  suddenly 
thickened  at  the  nodes  of  Ranvier,  the  spindle-shaped  en- 
largement being  apparently  due  to  an  increase  in  the 
amount  of  neuroplasm  with  an  accompanying  separation 
of  the  fibrillae.  Some  histologists  maintain  that  the  axis 
cylinder  is  invested  with  a  delicate  structureless  sheath, 
for  which  Kuehne  has  proposed  the  name  of  the  axlleaiina  : 
whether  this  structure  exists  in  the  living  fibre  is  still  a 
matter  of  question. 

'  The  medullary  sheath  also  presents  (under  certain  treat- 
ment) apparent  evidence  of  a  structure  that  would  hardly 
be  inferred  from  its  semifluid  character  as  seen  in  the  fresh 
nerve.  A  reticular  framework  of  a  substance  of  a  horny 
nature  known  as  neurokeratin  can  be  demonstrated, 
whose  meshes  and  filaments  vary  greatly  in  size  in  differ- 
ent parts  of  the  same  fibre.  That  the  substance  in  ques- 
tion exists  as  a  component  of  myelin  is  probably  true:  but 
the  solid  framework  described  is  quite  possibly  due  to  its 
coagulation  by  the  reagents  employed.  Far  more  con- 
spicuous are  the  oblique  clefts  seen  in  the  medullary  sheath 
after  treatment  with  certain  reagents,  notably  osmic  acid  : 
these  are  evidently  the  view  in  section  of  conical  cleavage 
spaces  running  from  the  primitive  sheath  to  the  axis 
cylinder,  and  dividing  the  medullary  sheath  into  the  me- 
dullary segments  of  Schmidt  and  Lantermann,  a  number 
of  which  may  be  found  ineachinternode:  whether  these  are 
real  or  artificial  must,  however,  be  regarded  as  still  unset- 
tled. 


,-A.I,  f*^' 


CHAPTKH    X.      NKRVOTTS   TISSIKS.  109 

The  iiciirik'iiinia  exhibits  no  special  sLniclur.'il  features 
worthy  of  remark.  It  should  he  noted  that  when  a  me- 
dullnted  fibre  joins  the  brain  or  cord,  while  the  medullary 
sheath  is  continued  within  the  axial  structure  as  far  as 
the  gray  matter,  the  neurilemma  disappears:  thus  the 
columns  of  the  cord  are  made  up  in  f^^reat  measure  of  me- 
dullated  fibresdev^oid  of  neurilemma.  At  the  distal  extremi- 
ties of  the  fibres  the  medullary  sheath  is  the  first  to  disap- 
pear, the  neurilemma  bcinj^  continued  for  some  distance 
toward  the  terminal. 

Nerve  fibres,  both  gray  and  meduUated,  var}'  considera- 
bly in  size,  their  diameters  ranging  from  two  to  twenty 
micra  ;  the  difference  appears  to  be  associated  with  a  cor- 
responding difference  in  the  length  of  the  fibres. 

Corpuscles,  fibres,  and  terminals  are  now  known  to 
be  continuous  structures  and  components  of  what  may 
properly  be  called  true  tissue  elements,  meaning  by  that 
term  in  each  case  the  result  of  the  modification  of  a  single 
embr3''onic  cell.  As  indicated  at  the  outset,  such  an  element 
may  consist  of  a  receiving  terminal,  an  afferent  fibre 
(medullated  or  nonmedullated),  a  central  corpuscle,  an 
efferent  fibre  (of  either  kind)  and  a  discharging  terminal. 
The  simplest  form  of  terminal  is  in  either  case  a  tuft  of 
fibrillae:  if  the  subdivisions  of  the  receiving  terminal  are 
called  dendrites,  and  the  discharging  cluster  an  arboriza- 
tion, the  two  can  readily  be  distinguished  by  these  terms. 
A  corpuscle  so  situated  would  be  essentially  bipolar;  such 
corpuscles  exist,  though  not  in  great  numbers,  in  the  ner- 
vous tissues  of  the  higher  vertebrates :  more  frequently 
the  points  of  attachment  of  the  two  fibres  become  approx- 


110  PARTI.      THE  TISSUES. 

iniated  and  finally  consolidated  for  a  short  distance,  form- 
ing what  is  apparently  a  unipolar  corpuscle  with  what  is 
termed  either  a  Y-  or  a  T-connection  according  to  the 
mode  of  separation  of  the  two  fibres.  In  certain  super- 
ficially situated  elements  of  a  sensory  character  in  some  of 
the  lower  animals  (and  possibly  in  higher  forms  as  well) 
the  receiving  terminal  and  afferent  filament  become  so 
shortened  and  condensed  as  to  form  a  mere  eminence  onl\' 
on  the  body  of  the  corpuscle:  such  elements  may  be  said 
to  be  in  form  (but  even  then  not  in  function)  unipolar. 
What  have  been  called  in  the  past  apolar  corpuscles  prob- 
ably do  not  exist. 

In  the  ganglia  of  the  sympathetic  system  corpuscles  are 
found  with  more  than  two  processes,  each  of  which  be- 
comes an  axis  cylinder  (or  a  gray  fibre) :  such  corpuscles 
are  in  the  strictest  sense  multipolar:  whether  the  majority 
of  the  poles  are  afferent  or  efferent  is  unknown :  both 
conditions  may  possibly  occur. 

The  term  multipolar  has  long  been  applied  to  the  cor- 
puscles found  chiefly  in  the  brain  and  spinal  cord  in  which 
a  distinction  can  be  made,  as  has  been  pointed  out,  be- 
tween a  single  axis-cylinder  process  and  a  number  of  so 
called  protoplasmic  processes  which  subdivide  into  a 
group  of  dendrites.  It  has  been  suggested  that  the  latter 
have  for  their  function  some  connection  with  the  nutrition 
of  the  corpuscle:  but  a  more  reasonable  interpretation  is 
one  which  regards  such  a  corpuscle  as  resulting  from  the 
disappearance  of  the  afferent  fibre,  its  primary  subdivi- 
sions thus  becoming  processes  of  the  corpuscle  itself.  The 
axis  cylinder  process  may  in  its  course  give  off  one  or  more 


ClIAPTKK    X.      NKUVOUS   TISSITRS.  Ill 

slciidcr  branches ;  these  leave  the  process  at  well  marked 
angles,  but  soon  after  bend  str()n<^ly  to  become  ajjproxi- 
matcly  parallel  to  it  In  most  cases:  they  are  known  as 
collaterals,  and  like  the  processes  from  which  they  arise 
terminate  in  arborizations. 

Axis-cylinder  processes  which  pass  from  the  jj^ray  into  the 
white  matter  of  the  cord  become  invested  with  a  medul- 
lary sheath  and  are  then  true  axis  cylinders:  elements  in 
which  this  is  the  case  are  known  as  corpuscles  of  the  first 
type:  in  other  cases  the  efferent  process  is  (piite  short,  the 
terminal  arborization  beinr^  situated  in  the  gray  matter: 
such  elements  are  called  corpuscles  of  the  second  type. 
The  disappearance  of  the  process  altogether,  making  the 
arborization  sessile,  like  the  dendrites,  gives  rise  to  the 
amacrine  corpuscles  of  Cajal. 

Nerve  corj)uscles  alwa\^s  have  large  and  conspicuous 
nuclei,  in  the  vicinity  of  which  a  patch  of  pigment  granules 
is  very  commonly  present.  The  iibrillae  of  the  processes 
may  be  traced  into  the  interior  of  the  corpuscles,  but  their 
internal  distribution  is  as  yet  unknown.  The  corpuscles 
are  almost  always  situated  in  well  defined  lymph  spaces 
which  agree  closely  with  them  in  contour.  The  forms  of 
the  corpuscles  of  the  brain  and  cord  will  be  described  in 
the  chapter  devoted  to  those  organs. 

The  nerves  are  definite  aggregates  of  nerve  fibres :  like 
the  blood  vessels,  the\'  penetrate  the  organs  of  the  bod\' 
and  are  consequently  to  be  regarded  among  the  factors  of 
structure  thereof:  the  same  is  true  of  manv  sfanjrlia:  both 
will  therefore  be  described  at  this  time. 


112  PART  I.      THE  TISSUES. 

A  nerve  is  a  bundle  of  nerve  fibres  or  an  aggregate  of 
such  bundles.  Each  bundle  is  termed  a  funiculus,  and  is 
composed  of  a  number  of  fibres  surrounded  by  a  cjUndrical 
sheath  called  the  perineurium.  The  latter  is  lamellated 
in  structure,  the  number  of  lamellae  never  being  less  than 
three  save  in  the  smallest  branches  of  the  ner  v^es :  they  are 
separated  by  distinct  lymph  spaces  lined  with  endothelioid 
corpuscles.  The  inner  lamella  is  continued  into  the  funi- 
culus by  the  connective  tissue  which  lies  between  the  fibres 
and  supports  their  capillaries,  called  the  endoneurium. 
In  small  funiculi  this  connective  tissue  is  homogeneous  in 
composition,  approaching  gelatinous  tissue  in  consistency  : 
such  funiculi  are  termed  simple:  larger  funiculi,  called 
compound,  show  here  and  there  in  the  endoneurium  con- 
nective tissue  septa  which  divide  the  funiculus  irregularly. 

In  small  nerves,  consisting  of  but  a  single  funiculus,  the 
outer  lamella  of  the  perineurium  is  continuous  with  the 
adjacent  areolartissue:  where  several  or  more  bundles  are 
associated,  however,  as  in  the  larger  nerves,  a  definite 
mass  of  connective  tissue,  containing  more  or  less  fat,  and 
definitely  compacted  on  its  outer  surface,  invests  and  sup- 
ports the  funiculi,  becoming  continuous  with  their  outer 
lamellae:  this  is  known  as  the  epineurium.  Within  it 
the  associated  funiculi  divide  and  anastomose  from  time 
to  time,  each  large  nerve  being  thus  in  reality  a  greatly 
elongated  plexus. 

As  the  funiculi  divide  into  small  groups  of  fibres  and 
finally  into  single  fibres  in  the  vicinity  of  their  destination, 
the  perineurium  becomes  greatly  reduced,  being  finally 
continued  for  a  short  distance  on  the  single  fibres  either  as 


CIIAPTI^R    X.      Ni;UV()l'S   TISSUES.  113 

a  single  lamella  or  as  a  mere  layer  of  etidothelioid  eclls : 
sneli  an  investment  is  known  by  the  name  of  Henle's 
sheath. 

A  ganglion  is  a  mass  of  nerve  corpuscles  invested  with 
a  definite  sheath  or  capsule  of  connective  tissuecontinuous 
with  the  e|)ineurium  of  the  nerv^cs  with  which  it  is  asso- 
ciated; or,  in  the  case  of  nerves  consisting  of  single  funi- 
culi, with  the  perineurium.  These  ma}^  be  but  two  in 
number,  the  ganglion  in  such  cases  being  practically  seated 
upon  a  nerve  trunk;  or  there  maybe  three  or  more,  the 
!2fanorli<jn  being  situated  at  their  intersection.  In  almost 
all  the  larger  ganglia  there  may  be  clearly  distinguished  a 
cortical  portion,  consisting  chiefl}'^  of  nerve  corpuscles,  and 
a  central  portion,  consisting  largely  of  nerve  fibres.  Of 
these  some  pass  directly  through  the  ganglion,  while  others 
pass  into  or  out  from  thecortical  portion,  being  connected 
with  the  cori)uscles.  Each  corpuscle  is,  as  a  rule,  con- 
tained in  a  delicate  capsule  continuous  with  the  neuri- 
lemma of  the  associated  fibre  or  fibres  and  enclosing,  as 
has  already  been  stated,  a  pericorpuscular  lymph  space. 

The  neuroglia,  or  sustentacular  tissue  of  the  brain  and 
cord,  has  been  referred  to  in  the  chapter  devoted  to  the 
fibrous  tissues.  It  differs  from  all  the  tissues  of  that  group 
in  its  origin  and  in  the  absence  of  anythinglike  the  matrix 
characteristic  of  them  ;  consisting,  as  was  stated,  entirely 
of  peculiar  branched  corpuscles  known  as  glia-cells. 
These  are  stellate  or  irregularly  shaped  cells  with  large 
nuclei,  which  stain  conspicuously  with  some  reagents. 
Their  branches,  which   are  quite  numerous,  terminate  in 


11 4  PART   I.      THE   TISSUES. 

long  slender  processes :  these  are  stated  bv  Ranvier  to  be 
fibrillated,  the  fibrillae  passing  through  the  body  of  the 
cell  from  one  process  to  another:  the\'  are  varioush^  ar- 
ranged upon  glia-cells  from  different  parts  of  the  cerebro- 
spinal axis,  those  of  the  gray  matter  of  the  cord,  for  exam- 
ple, differing  from  those  of  the  white  in  the  number  and 
disposition  of  their  processes.  In  a  general  way  it  may 
be  said  that  the  latter  form  a  dense  reticulum  of  closely 
interwoven  fibres:  this  in  sections  has  a  fineh^  granular 
appearance  conspicuously  seen  in  the  gray  matter.. 

The  glia-cells  are  found  in  the  cerebrospinal  axis  and  are 
derived  from  the  same  embryonic  la^^er  as  the  nervous  ele- 
ments themselves.  While,  therefore,  their  function  is 
probably  purely  mechanical,  or,  in  a  sense,  skeletal,  they 
must  be  regarded  as  closelv  related  to  the  nervous  tissues 
rather  than  to  the  skeletal  tissues  proper.  These  latter 
also  penetrate  the  cerebrospinal  axis,  in  the  form  of  con- 
nective-tissue trabeculae  which  compose  a  proper  skeletal 
framework :  and  although  they  gradually  diminish  by 
subdivision  and  become  reduced  to  delicate  fibrils  inter- 
mingled with  those  of  the  neuroglia,  nothing  like  a  transi- 
tion from  one  tissue  to  the  other  has  ever  been  observed. 

^"^     ^  ^     II. /^  -     /  t 


V 


OLyw|-^^ 


en  APTHR  XI.   STRUCTURE  OF  TIIH  CKLL.     115 


CHAPTER  XI. 
THE  STRUCTURE  OF  THE  CELL. 


\Vc  have  now  passed  briefly  in  review  the  various  tissue 
elements,  considering  both  their  form  and  characters  and 
their  union  to  compose  the  tissues  of  the  body;  as  also  the 
structure  of  some  of  the  simpler  aggregates  of  tissues,  or 
organs.  The  elements  of  the  tissues  were  at  the  outset 
defined  as  cells  or  as  derived  from  the  modification  of  cells; 
and  a  cell  was  defined  as  a  nucleated  mass  of  protoplasm. 
It  is  important  now  for  us  to  consider  the  structure  of  the 
protoplasm  itself,  and  of  the  nucleus  as  well;  and  to  learn 
something  of  the  process  by  means  of  which  new  cells  are 
formed. 

It  was  stated  in  the  opening  chapter  that  the  protoplasm 
which  makes  up  the  body  of  the  cell  is  neither  homogene- 
ous or  structureless,  as  it  was  once  supposed  to  be.  The 
delicate  granulation  generally  characteristic  of  its  appear- 
ance as  seen  by  ordinary'  powers  proves  with  more  im- 
proved means  of  research  to  be  the  expression  of  a  delicate 
reticulum  or  network  of  a  somewhat  denser  substance 
which  has  been  given  the  name  of  spongioplasm ;  its 
meshes  are  filled  with  a  less  dense  or  semifluid  substance 
designated  as  hyaloplasm:  the  proportion  between  the 
amounts  of  these  two  substances  may  var^^  greatly:  as  a 


116  PART   ].      THE   TISSUES. 

i^^enerjil  rule  the  relative  amount  of  the  former  increases 
with  the  age  of  the  cell.  The  meshes  of  the  spongioplasm 
may  var^y  greatly  in  size  and  in  the  coarseness  or  fineness 
of  their  constituent  fibrils:  the  accumulations  at  their  in- 
tersections are  the  granules  most  readily  seen. 

In  cells  which  become  surrounded  b}'  a  cell  wall  com- 
posed of  some  formed  product  (e.  g.,  an  epidermal  cell), 
the  reticulum  becomes  quite  close  and  dense  near  the  sur- 
face:  but  in  man\'  cases  (e.  g.,  a  leucocyte)  the  converse  is 
the  case,  the  exterior  of  the  cell  consisting  almost,  if  not 
quite  wholly,  of  hyaloplasm.  Such  a  clear  outer  portion  is 
sometimes  termed  ectoplasm,  in  distinction  from  the  gran- 
ular inner  portion  known  as  endoplasm :  the  distinction 
is,  however,  of  questionable  value,  since  in  some  of  the 
lowest  animals  the  conditions  are  reversed,  the  same 
terms  being  applied  to  a  denser  outer  and  a  more  fluid 
inner  portion  of  the  cell.  The  terms  paraplasm  and  deu- 
toplasm  are  also  sometimes  made  use  of  in  connection 
with  the  structure  of  the  cell  body  to  designate  granules 
imbedded  in  the  protoplasm,  and  consisting  either  of  sub- 
stances taken  up  by  the  protoplasm  in  aj^olid  form,  or  of 
formed  products  temporarily  stored  in  the  cell ;  such,  for 
example,  as  yolk  granules  in  the  ovum. 

The  nucleus  gives  evidence  of  a  reticular  structure  even 
with  ordinary  powers ;  and  this  structure  is  also  clearly 
seen  to  vary  in  the  different  nuclei  of  adjacent  cells  or  in  the 
same  cell  at  different  times  if  watched  while  still  living. 
Under  ordinary  conditions,  however,  the  nucleus  when 
seen  in  what  is  usually  designated   the  "resting"  condi- 


CHAI'THR    XI.      STUrCTl'KK    OF    TUT    CKI-L.  117 

lion  is  ri  splKToicial  vt'siciilar  body  hounded  \)y  a  well  de- 
fnicd  wall,  and  eontainiiiLi  the  network  al)ove  referred  to: 
this  is  sometimes  fnie  and  close  meshed;  at  others  com- 
posed of  but  a  few  coarse  fibrils,  which  are  in  some  cases 
quite  irrcf^^ularly  disposed  ;  in  some  cases  the  fibrils  form 
a  continuous  filamen^which  is  arran^red  in  a  tangled 
skein.  The  nodes  of  the  network  in  many  cases  form 
coarse  granules,  which  are  cpiite  conspicuous  :  in  addition 
there  are  often  seen  in  the  nuclei  distinct  spheroidal  bodies 
apparently  different  in  composition  from  the  network: 
such  a  bod}'  is  called  a  nucleolus. 

The  meshes  of  the  network  are  filled  with  a  clear  semi- 
fluid substance  which  is  not  readily  colored  by  the  stain- 
ing fluids  wdiich  render  the  network  and  nucleoli  conspicu- 
ous. This  difference  between  the  two  principal  substances 
of  the  nucleus  led  Flemming  to  propose  for  the  substance 
composing  the  filaments  the  name  of  chromatin,  and  for 
the  clear  substance  that  of  achromatin.  More  recent  re- 
searches have  made  clear,  however,  the  fact  that  the 
denser  portion  of  the  nucleus  itself  consists  in  part  of  a 
substance  which  does  not  stain  any  more  readily  than  the 
more  fluid  portion:  while  the  latter,  now  usually  termed 
the  nuclear  matrix,  is  regarded  as  possibly  similar  in  com- 
position to  the  hyaloplasm  of  the  cell-body.  The  name  of 
chromoplasm  has  therefore  been  proposed  by  Carnoy  for 
the  substance  which  forms  the  filaments,  that  portion 
which  is  readily  stained  being  designated  by  the  term 
chromatin,  and  that  which  resists  ordinary  stains  by  the 
term  achromatin;  a  different  application  of  these  terms 
from  that  originally  i)roposed  by  Flemming. 


lis  PART   I.      THE   TISSUES. 

The  nuclear  wall  is  composed  of  chromoplasm  :  by  some 
it  is  regarded  as  consisting  merely  of  a  fine  and  close  net- 
work of  that  substance;  b\'  others  as  a  definite  and  con- 
tinuous layer.  Its  continuity  with  the  network  is  evident 
from  its  comportment  at  the  time  of  nuclear  division. 
Whether  the  nucleoli  consist  of  chromoplasm  must  for  the 
present  be  regarded  as  an  open  question. 

While  the  arrangement  of  the  filaments  of  the  nuclear 
network  is  often  exceedingly  irregular,  especially  in  the 
resting  stao^e,  it  can  often  if  not  alwavs  be  seen  at  the 
time  of  its  greatest  development  to  have  a  definite  plan, 
whose  basis  is  the  formation  of  a  larger  or  smaller  num- 
ber of  elongated  loops  having  a  meridian-like  arrange- 
ment in  relation  to  a  definite  axis.  The  turns  of  the  loops 
are  directed  toward  one  end  of  the  axis,  termed  the  pole 
of  the  nucleus:  the  free  extremities  meet  (and  frequently 
interdigitate)in  the  region  around  the  opposite  end  of  the 
axis,  the  anti-pole.  The  sides  of  the  loops  are  often  ex- 
ceedingly irregular  in  their  course,  and  may  in  addition 
branch  frequently,  the  branches  anastomosing  and  thus 
forming  the  irregular  netw^ork  commonly  seen.  Where 
the  ends  of  adjacent  loops  become  continuous  the  convo- 
luted filament  sometimes  seen  is  produced. 

The  division  of  older  cells  to  form  new  ones  is  preceded 
in  man  and  the  higher  animals  generally  by  nuclear 
division.  The  older  observers  (whose  imperfect  micro- 
scopes showed  them  in  the  interior  of  the  nucleus  only  the 
nucleoli  and  the  coarser  nodal  granules)  believed  this  pro- 
cess to  be  quite  simple  in  its  nature,  consisting  merely  in 


CHAPTKK    XI.      STRrCTVRE   OF  THK   CELL.  119 

the  passage  of  a  clcavaijc-planc  throuijjh  the  nucleus  in  the 
same  wav  as  is  seen  in  the  cell  body  itself.  This,  which  is 
called  direct  division,  may  possibly  sometimes  occur;  but 
the  constant  advance  of  our  knowledge  makes  it  yearly 
more  and  more  evident  that  the  common  mode  of  nuclear 
division  as  it  occurs  in  jilants  and  animals,  and  in  normal 
and  i)athological  changes  alike,  is  an  exceedingly'  complex 
process,  to  which  the  rather  unfortunate  name  of  indirect 
division  is  commonly  applied :  and  it  is  quite  possible,  if 
not  probable,  that  this  is  the  sole  method.  The  series  of 
changes  involved  in  this  process  has  been  termed  karyo- 
kinesis.  The  name  of  mitotic  division  has  also  been  ap- 
plied to  the  process,  the  successive  stages  being  called 
mitoses:  the  so-called  direct  division  being  distinguished 
as  amitotic. 

The  successive  steps  that  can  be  recognized  in  what  is 
really  one  continuous  process  have  been  designated  by 
special  names,  based  on  the  appearances  presented  b\'  the 
nucleus  from  time  to  time.  The  first  step  is  the  formation 
of  what  has  been  variousl}'  termed  the  spirem,  or  close 
skein :  the  secondary  filaments  are  retracted  into  the 
primary  filaments  or  loops;  the  nuclear  membrane  is  also 
absorbed,  as  are  the  nucleoli;  the  latter  fact  indicating  the 
possible  identity  of  these  bodies  with  the  chromoplasm  of 
the  network.  The  next  step  is  the  formation  of  the  open 
skein,  or  wreath :  the  j^rimary  filaments  contract,  be- 
coming shorter  and  stouter,  and  having  a  less  tortuous 
course:  they  gradually'  assume  the  form  of  an  equatorial 
wreath  of  loops  with,  as  has  been   stated,  their  flexures 


120  PART  I.      THE  TISSUES. 

turned  toward  the  region  of  the  pole.     To  these  loops  the 
name  of  chromosomes  has  been  given. 

While  the-chromosomes  are  thus  being  defined,  there  ap- 
pears in  the  polar  area  a  group  of  fibres  of  achromatin, 
known  froin  its  form  by  the  name  of  the  achromatic  spin- 
dle: this  gradually  moves  toward  thecentreof  thenucleus, 
taking  an  axial  position  with  its  extremities  directed 
toward  the  pole  and  the  anti-pole  respectively.  The  chro- 
mosomes having  b\^  this  time  become  quite  short  and 
stout,  and  V-shaped  from  the  divergence  of  the  limbs  of 
the  loops,  attach  themselves  to  the  spindle,  eventually  as- 
suming a  radial  position  at  its  equator.  When  the  nucleus 
is  viewed  at  this  stage  from  a  polar  or  an  anti-polar  direc- 
tion the  radiating  arms  of  the  chromosomes  together  form 
a  starlike  figure  to  which  the  name  of  the  aster  has  been 
given. 

During  the  formation  of  the  aster  or  immediateK^  there- 
after, the  cleavage  of  the  chromosomes  take  place,  each 
loop  being  split  into  two  similar  (but  of  course  more 
slender)  loops  by  a  plane  passmg  through  them  all  equa- 
torially.  This  is  the  central  process  in  the  division  of  the 
nucleus:  the  changes  which  follow  have  therefore  been 
sometimes  designated  by  the  term  metakinesis:  they  are 
in  a  certain  sense  the  retracing  of  the  processes  already  de- 
scribed. In  its  more  limited  sense  the  term  metakinesis  is 
applied  only  to  the  cleavage  of  the  chromosomes,  and  the 
changes  immediately  connected  therewith. 

The  daughter-loops  formed  by  the  cleavage  of  the  chro- 
mosomes begin  to  separate  first  at  their  apices,  these  being 
turned  toward  the  extremities  of  the  spindle:  they  then 


CHAPTER     XI.      STKICTIKE   OF   THE   CELL.  121 

travel  slowly  along  the  achromatic  til)rcs  in  each  direc- 
tion, two  sets  of  chromosomes  thus  being  formed;  these 
gradually  arrange  themselves  about  the  poles  of  the  spin- 
dle, which  have  now  become  the  polar  areas  of  the  two 
daughter-nuclei.  The  limbs  of  associated  loops  remain  for 
some  time  connected  together  by  delicate  achromatic 
uniting  filaments:  the  whole  figure  seen  from  the  side  has 
a  resultant  barrel-shaped  appearance ;  at  either  extremity 
the  chromosomes  have  a  stellate  arrangement,  and  this 
stage  is  therefore  designated  the  dyaster. 

As  the  dyaster  is  formed,  a  cleavage  plane  passes  through 
the  body  of  the  cell,  whose  course  in  the  nuclear  region  is 
sometime  marked  by  nodal  points  on  the  uniting  filaments. 
As  their  separation  is  completed,  the  free  extremities  of  the 
chromosomes  of  the  daughter-nuclei  bend  inward  toward 
the  new  antipolar  areas,  which  face  the  plane  of  cleavage 
and  therefore,  also,  each  other.  Changes  now  ensue  in 
the  inverse  order  of  those  described  as  taking  place  at  the 
beginning  of  the  process,  the  chromosomes  becoming  first 
converted  into  open  skeins,  and  later  into  the  closed 
skeins  whose  farther  modification  gives  rise  in  each  to  a 
reticulum  b\'  the  formation  of  anastomosing  secondary- 
filaments,  accompanied  with  or  followed  by  the  formation 
of  a  nuclear  membrane,  the  appearance  in  some  cases  of 
nucleoli,  and  the  final  assumption  of  the  resting  stage. 
The  stage  in  which  two  adjacent  and  parallel  skeins  are 
seen  is  sometimes  termed  the  double  skein  or  dispirem : 
but  it  is  evident  that  w'e  are  here  dealing  with  structures 
which,  though  genetically  associated,  are  now  parts  of 
Separate  and  distinct  nuclei. 


122  PART  I,      THE  TISSUES. 

The  number  of  the  chromosomes  varies  considerably  in 
different  plants  and  animals,  but  is  probably  constant  for 
the  same  tissues  in  each  species :  their  form  may  also  vary 
greatly,  particularly  as  regards  the  length  of  the  branches 
of  the  loops.  The  changes  above  described  may  in  many 
cases  be  followed  step  by  step  with  a  good  microscope  of 
ordinary  powers,  either  as  they  take  place  in  living  cells, 
or  as  thc}^  may  be  found  in  adjacent  cells  of  suitably  pre- 
pared tissues,  each  of  the  characteristic  figures  mentioned 
being  clearly  recognizable :  in  some  instances,  however, 
owing  to  the  irregularity  in  the  form  of  the  chromosomes, 
or  to  variations  in  the  rate  of  their  transformation,  some 
of  the  phases  may  be  so  far  modified  as  to  be  no  longer 
distinguishable.  The  essential  features  of  the  process  are, 
nevertheless,  always  to  be  discerned,  and  should  be  dis- 
tinctly borne  in  mind  :  they  are,  in  their  order,  as  follows  : 
first,  the  collection  of  the  chromatin  of  the  nuclear  net- 
work into  chromosomes  ;  second,  the  equatorial  arrange- 
ment of  the  latter  in  what  has  been  termed  the  nuclear 
plate;  third,  their  metakinetic  cleavage ;  fourth,  the  sepa- 
ration of  the  two  sets  of  chromosomes  thusformed  ;  and 
finally  their  resolution  into  the  nuclear  networks  of  the 
two  resultant  daughter-nuclei,  whose  formation  b}'^  this 
method  is  accompanied  by  the  cleavage  of  the  protoplas- 
mic body  of  the  parent  cell,  thus  completing  the  forma- 
tion of  new  cells. 

It  has  recently  been  made  fully  evident  that  the  changes 
taking  place  in  the  chromatin  of  the  nucleus  during  karyo- 
kinesis  are  accompanied  and  preceded  by  other  equally 
complicated  changes  in  structures  made  up  wholly  of  ach- 


CHAPTKK    XI.      STKICTI'KK    OF   THK   CKLL 


12vS 


roniatin,  the  formation  of  the  achromatic  spituUc  hcin<(  a 
jiortion  thereof.  While  these  changes  are  doubtless  as  im- 
portant as  those  already  described,  their  nature  is  as  yet 
far  less  clearly  understood.  A  description  of  them  ww 
therefore  not  be  necessary  at  this  time. 


END   OF    PART   I. 


'  e' 


Hi 


9   I 


PART    II. 


HISTOLOGICAL  ANATOMY. 


CHAPTER    XII.      INTRODT'CTORY.  127 


CHAPTER  XII. 
INTRODUCTORY. 


HistolDgical  Anatomy  has  already  been  defined  as  the 
study  of  the  arrangement  of  the  tissues  to  form  the  organs 
of  the  body :  an  organ  has  also  been  defined  as  a  particu- 
lar part  of  the  body  having  a  definite  form  and  function : 
and  it  is  a  familiar  fact  that  organs  having  common  or 
essentially  similar  functions  are  associated  together  under 
the  name  of  a  system,  whether  they  are  continuous,  as  in 
the  case  of  the  nervous,  or  discontinuous,  as  in  that  of  the 
muscular  system. 

While  the  study  of  the  histological  anatom\'  of  the 
organs  can  in  most  cases  be  pursued  most  naturally  by 
considering  them  in  their  relations  as  components  of  the 
various  physiological  systems,  on  account  of  the  commu- 
nity of  structure  which,  in  most  cases,  characterizes  asso- 
ciated organs,  and  while  structure  and  function  are  with- 
out question  closely  (though  not  always  evidenth-)  re- 
lated, it  should  always  be  kept  clearly  in  mind  that  we  are 
here  concerned  with  structure  only  ;  and  particularly  with 
structure  as  composed  of  tissues :  our  constant  endeavor 
should  be,  in  the  first  place,  to  analyze  the  organs  into  the 
tissues  of  which  they  are  composed,  and  to  determine  the 
relations  of  each  to  the  others ;  and  in  the  second  place  to 
note  any  characteristic   pecidiarities  exhibited  by  any  of 


128  PART    II.      HISTOLOr.ICAL  ANATOMY. 

the  tissues  present ;  this  should  be  accompanied  in  each  in- 
stance by  a  careful  consideration  of  the  disposition  and 
characters  of  the  compound  factors  of  structure  present, 
such  as  the  blood,  lymph,  and  nervous  supply. 

Tissues  have  alread}'-  been  defined  as  masses  of  cells  or 
of  cell-derivatives;  and  since  tissues  compose  the  organs, 
and  organs  make  up  the  whole  body,  it  follows  that  the 
body  is  to  be  regarded  as  a  mass  of  more  or  less  modified 
cells.  The  innumerable  cellular  elements  which  make  up 
the  adult  human  organism  are  in  every  case  derived  from 
the  division  of  previously  existing  cells,  and  are  therefore 
necessarily  the  descendents  of  a  single  ancestral  cell.  That 
cell  is  the  fertilized,  ovum  or,  as  it  is  sometimes  termed, 
the  oosperm.  The  study  of  fertilization,  of  the  segmenta- 
tion of  the  oosperm,  and  of  the  subsequent  development  of 
the  tissues  and  organs  of  the  body  lie  strictly  within  the 
province  of  the  science  of  Embryology' ;  but  a  brief  state- 
ment of  the  origin  of  the  various  tissues  may  with  advan- 
tage be  given  here,  as  throwing  light  on  the  structure  and 
relations  of  the  organs  of-the  body. 

Repeated  cell  division  or  segmentation  in  a  short  time 
divides  the  oosperm  into  a  spheroidal  mass  of  apparently 
similar  cells :  these  soon  arrange  themselves  in  two  dis- 
tinct layers,  from  w^hich  a  third  intermediate  layer  is 
shortly  afterward  derived:  the  outer  of  these  layers  is 
called  the  epiblast  or  ectoderm,  the  middle  the  mesoblast 
or  mesoderm,  and  the  inner  the  hypoblast  or  entoderm; 
the  whole  trilaminar  structure  receiving  the  name  of  the 
blastoderm. 


CHAPTKR    XII.      INTRODUCTORY.  129 

From  the  cells  which  compose  the  epiblast  are  derived 
the  following  structures : 

The  epidermis,  and  its  appendages  the  hairs  and  the 
nails,  and  the  epithelium  lining  the  tegumentary 
glands  (sweat  glands,  sebaceous  glands,  mammary 
glands). 

The  epithelium  of  the  nasal  passages  and  the  asso- 
ciated cavities  and  glands. 

The  epithelium  of  the  mouth  and  of  the  glands  con- 
tinuous therewith,  and  of  a  portion  of  the  tongue: 
the  taste  organs :  the  enamel  of  the  teeth. 

The  epithelium  of  the  conjunctiva  and  of  the  glands 
of  the  eyelid,  and  of  the  front  of  the  cornea:  the 
lens  of  the  eye:  the  retina  (secondarih-  as  an  out- 
growth from  the  brain). 

The  epithelium  of  the  membranous  lab\'rinth  of  the 
ear. 

The  epithelium  lining  the  cavities  contained  in  the 
cerebrospinal  axis :  the  nervous  tissues :  the  neuro- 
glia :  the  pineal  body  :  the  pituitary  body. 

From  the  cells  which  compose  the  middle  layer  or  meso- 
blast  are  derived  the  following  structures: 

The  epithelium  of  the  urinary  and  genital  organs 
(with  the  exceptions  of  the  epithelium  of  the  blad- 
der and  urethra),  including  the  reproductive  ele- 
ments of  both  sexes. 

All  the  muscular  tissues  of  the  body,  with  the  excep- 
tions of  the  cells  (doubtfully  muscular)  found  in 
the  sweat  glands. 

The  skeletal  tissues  of  all  sorts  throug^hout  the  bod  v. 


130  PART    II,      HISTOLOGICAL  ANATOMY. 

The    bloo(J-vascular    and    lymph-vascular    system : 
the  serous  membranes :  the  spleen  and  other  adenoid 
bodies:  the  blood  and  lymph  corpuscles. 
It  should  be  stated  that  by  some  histologists  the  cells 
which  give  rise  to  the  tissues  of  the  first  two  groups  are 
regarded  as  having  an  origin  somewhat    different    from 
those  giving  rise  to  the  last  two :  the  name  mesoblast  has 
been  retained  by  them  as  a  collective  title  for  the  former, 
while  for  the  latter  the  name  of  parablast  was  proposed 
bj^  His,  and  later  that  of  mesenchyma  by  the  Hertwigs. 
While  this  is  probably  true  of  the  tissues  of  birds,  it  has 
not  yet  been  proven  for  any  mammal:  and  there  are  spe- 
cial reasons  why  it  might  be  true  in  one  case  and  not  in 
the  other. 

From  the  cells  which  compose  the  hypoblast  are  derived 
the  following  structures : 

The  epithelium  of  the  back  of  the  tongue,  the  lower 
part  of  the  pharynx,  the  oesophagus,  stomach  and 
intestines :  that  of  all  the  glandular  appendages  of 
the  alimentary  canal. 

The  epithelium  of  the  Eustachian  tube  and  middle 
ear. 

The  epithelium  of  the  larynx,  the  trachea,  the  bronchi, 
the  bronchial  tubes  and  the  air  sacs  of  the  lungs. 

The  epithelium  lining  the  urinary  bladder  and  the 
urethra. 

The  epithelium  lining  the  vesicular  alveoli  of  the  thy- 
roid body. 

The  concentric  corpuscles  or  epithelial  nests  of  the 
thymus. 


CHAPTER   XII.      INTRODUCTORY.  131 

Even  a  brief  study  of  the  tabularstatementof  the  origin 
of  the  tissues  of  the  body  above  given  will  make  clear  the 
facts  that  most  organs  are  made  up  of  tissues  derived  from 
more  than  one  of  the  primary  tissue-layers,  and  that  in 
some  cases  at  least,  tissues  which  are  structurally  continu- 
ous and  to  all  appearances  similar  arc  of  difterent  embry- 
onic origin  :  this  is  notably  the  case  with  the  transitional 
epithelium  found  in  the  ureters  and  the  bladder;  and  other 
instances  might  be  mentioned. 


The  order  of  study  pursued  in  acquiring  a  knowledge  of 
the  histological  anatom\'  of  the  various  organs  and  sys- 
tems of  the  body  is  plainly'  a  matter  of  convenience.  That 
which  will  be  here  pursued  is  one  shown  by  experience  to  be 
desirable  on  some  accounts :  but  it  should  be  understood 
that  it  may  be  readily  varied  at  will.  Omitting  from  far- 
ther consideration  the  simpler  organs  already  described  in 
the  preceding  part  in  connection  with  the  tissues  which 
chiefl\'  compose  them,  such  as  the  cartilages,  the  bones, 
the  muscles,  etc.,  the  various  regions,  systems  or  groups 
of  organs  will  be  successively  described  as  follows: 

The  tegumentary  system  will  first  receive  attention : 
this  includes  not  onh- the  skin,  but  also  those  solid  append- 
ages, the  hairs  and  the  nails,  which  are  derived  from  the 
special  modifications  of  its  outer  layer;  and  those  in- 
growths of  the  same  la\'er  which  constitute  the  sudor- 
iparous, sebaceous  and  mammary  glands. 


132  PART    II.      HISTOLOGICAL  ANATOMY. 

The  skin  upon  the  enter  surface  of  the  lips  is  continuous 
with  the  so-called  mucous  membrane  of  its  inner  surface : 
a  transition  which  brings  us  naturally  to  the  mouth  and 
its  contents:  this  includes  the  study  of  the  lining  mem- 
brane above  referred  to;  the  buccal  and  other  glands 
which  open  thereon ;  the  teeth  and  the  tongue. 

The  mouth  is  the  antechamber  of  the  alimentary  canal, 

though  it  is  often  regarded  as  a  part  of  it.  Beginning  with 
the  pharynx,  we  naturally  consider  in  their  order  the 
oesophagus,  the  stomach,  the  small  intestine  in  its  various 
regions,  and  thelargeintestine  (including  the  rectum ) ;  and 
also  the  glandular  appendages  of  the  canal,  the  liver  and 
the  pancreas. 

The  pharynx  is  not  only  a  portion  of  the  alimentary 
canal,  but  of  the  respiratory  tract  as  well :  the  latter 
being,  as  embryology  shows,  an  outgrowth  of  the  diges- 
tive tube ;  its  study  includes  that  of  the  larynx,  the  trachea 
and  bronchi,  the  bronchial  tubes  in  their  various  ramifica- 
tions, and  the  terminal  sacs  which  make  up  with  them  the 
proper  substance  of  the  lungs. 

The  bladder  and  the  urethra  are  parts  of  an  outgrowth 
from  the  posterior  region  of  the  alimentary  canal,  as  the 
respiratory  tract  is  of  the  anterior.  With  them  as  median 
structures  are  closely  associated  the  paired  organs  which 
complete  the  urinary  apparatus:  and  intimately  related 
therewith  are  the  male  and  female  reproductive  glands, 
and  the  accessory  organs  connected  with  reproduction 
and  micturition.    This  group  will  next  be  studied. 


CIIAPTKR    XII.      INTROI^rCTORY.  133 

Following  the  groups  of  organs  above  indicated,  the 
circulatory  system  may  next  receive  attention.  This  in- 
cludes the  study  of  the  heart,  the  larger  blood  vessels,  and 
the  greater  lymphatic  trunks,  the  smaller  blood  vessels 
and  lymphatics  having  been  already  considered.  Since 
the  great  serous  cavities,  such  as  that  of  the  thorax  or  of 
the  abdomen,  may  best  be  regarded  as  lymph  spaces,  the 
special  discussion  of  their  lining  membranes  may  appro- 
priateK'  be  considered  here. 

In  addition  to  the  lymph  nodes,  there  are  found  in  the 
body  larger  organs  apparently  allied  to  them  or  derived 
from  their  modification,  such  as  the  spleen  and  the 
thymus.  The  name  of  ductless  glands  has  long  been  ap- 
plied to  these  and  to  other  soft  organs  of  uncertain  func- 
tion, such  as  the  thyroid,  the  pituitary  body,  and  the 
adrenals  (the  so-called  suprarenal  capsules).  The  group 
(to  which  the  name  of  the  adenoid  bodies  is  sometimes  ap- 
plied) is  a  heterogeneous  one,  its  members  in  some  cases 
having  little  in  common  :  but  they  ma}'  for  convenience  be 
considered  together. 

The  structure  of  the  central  nervous  system  is  at  pres- 
ent being  worked  out  chiefly  b\' histological  methods:  it 
is  not  as  yet  alwaj's  easy  to  distinguish  between  what 
may  be  regarded  as  physiological,  and  what  as  histologi- 
cal anatomy  in  some  cases.  Imperfect  as  our  knowledge 
is  at  present,  the  briefest  statement  of  its  details  would 
transcend  the  limits  of  an  elementarj^  course  in  histo- 
logy ;  but  some  knowlede  of  itgs  most  salient  features  is 


134  PART    II.      HISTOLOGICAL  AXATOMY. 

essential:  the  structure  of  the  spinal  cord  will  be  discussed, 
together  with  that  of  the  principal  regions  of  the  brain. 
On  account  of  their  intimate  relation  to  the  cerebrospinal 
axis,  the  membranes  w^hich  invest  it  and  which  form  the 
lining  of  the  spinal  canal  will  be  taken  up  in  this  con- 
nection. 

Finall}'^  the  study  of  the  central  nervous  axis  ma}^  prop- 
erh^  be  followed  by  that  of  the  complex  outlj'ing  struc- 
tures which  are  the  essential  parts  of  the  organs  of  special 
sense,  such  as  the  eye,  the  ear  and  the  nose.  Beginning 
our  course  with  the  common  investment  of  the  body,  we 
close  it  with  organs  w^hich  are  in  a  great  measure  speciali- 
zations thereof. 


'-ci 


'^" 


AXA  trr 


CHAPTER    XIII.      SKIN   AND   APPENDAGES.  135 


CHAPTER  XIII. 

THE  SKIN  AND  APPENDAGES. 


The  skin,  which  is  the  investing  and  protecting  mem- 
brane of  the  bod}^  is,  like  other  membranes  found  upon 
free  surfaces,  composed  of  two  primary  layers,  one  of  which 
is  epithelial,  the  other  skeletal :  each  of  them  being  capable 
of  division  into  more  or  less  well  marked  secondary  layers 
or  strata.  The  outer  or  epithelial  la^'^er  is  known  variously 
as  the  epidermis,  cuticle,  or  scarf  skin :  the  inner  or  skeletal 
layer  as  the  derma,  corium,  or  cutis  (or  less  properly  the 
cutis  vera  or  "true"  skin). 

The  epidermis  can  under  favorable  circumstances  be 
seen  even  with  the  naked  eye  to  be  made  up  of  two  distinct 
layers,  an  inner  moist  or  mucous  layer  (sometimes  called 
the  rete  mucosum),  and  an  outer  dry  or  horny  layer; 
and  each  of  these  layers  can  be  resolved  by  the  microscope 
into  strata  characterized  by  differences  in  the  form  and 
arrangement  of  the  component  cells :  it  may  therefore  be 
regarded  as  the  best  example  of  a  stratified  squamous  epi- 
thelium in  the  whole  body.  Beginning  with  the  mucous 
layer,  we  find  next  the  corium  cells  which  are  columnar  in 
form,  and  which  are  constantl}'  undergoing  division ;  the 
new  cells  formed  at  their  free  extremities  are  at  first  verti- 
cally elongated,  then  polyhedral  in  shape,  and  later  some- 
what flattened  vertically :  they  form  with  the  basal  cells 


136  PART    II.      HISTOLOGICAL  ANATOMY. 

the  first  or  lowermost  stratum  of  the  epidermis,  known  as 
the  stratum  Malpighii :  the  elements  composing  this 
stratum  have  the  form  of  prickle-cells,  their  numerous 
short  processes  preventing  the  actual  contact  of  the  sur- 
faces of  adjacent  cells,  thus  forming  channels  for  the  circu- 
lation of  lymph  and  the  nutrition  of  the  elements :  all  the 
cells  of  this  stratum  may  be  regarded  as  living.  Here  and 
there  leucocytes  may  sometimes  be  found ;  they  have  wan- 
dered into  the  epithelium  from  below  and  occupy  irregular 
intercellular  spaces.  The  cells  of  the  deepest  portions  of  the 
stratum  Malpighii  contain  pjgment^ranules,  the  color  of 
the  skin  in  different  races  depending  chiefly  on  the  relative 
abundance  and  color  of  the  pigment.  Delicate  nerve  fibrils 
enter  this  stratum  of  the  epidermis,  and,  as  stated  in  a 
previous  chapter,  Merkel  has  described  special  terminals 
thereto  under  the  name  of  tactile  cells. 

As  new  cells  are  constantly  formed  in  the  deeper  por- 
tions of  the  stratum  Malpighii,  the  older  cells  are  as  con- 
stantly pushed  farther  and  farther  from  the  blood  vessels 
of  the  corium  which  constitute  their  basis  of  nutritive  sup- 
ply through  the  agency  of  the  lymph  channels  already 
mentioned.  At  a  certain  distance,  which  varies  in  differ- 
ent portions  of  the  body,  they  begin  to  yield  more  con- 
spicuously to  the  mechanical  pressure  from  without,  be- 
coming more  flattened  in  form,  and  at  the  same  time  to 
undergo  degenerative  changes,  granules  of  a  fat-like  com- 
pound termed  eleidin  appearing  in  their  substance  in  great 
numbers.  There  is  thus  formed  a  definite  layer  never  more 
than  a  few  cells  deep  to  which  the  name  of  stratum  gran- 
ulosum  is  applied:    the    stratum    granujosum    and    the 


CHAPTER    XlII.      SKIN    AM)   APPENDAGKS.  137 

Stratum  Malpigliii  together  make  up  the  mucous  layer, 
and  in  most  parts  of  the  body  the  greater  portion  of  the 
epidermis. 

ImmL'diately  above  the  stratum  granulosum  and  sharply 
distinguished  from  it  is  a  thin  layer  of  cells  which  resemble 
those  of  that  stratum  in  b:ing  compressed  in  form,  but 
differ  from  them  in  greater  homogeneity  of  substance  and 
therefore  in  transluccncy :  this  is  the  stratum  lucidum, 
the  lower  of  the  strata  of  the  horny  layer.  According  to 
Ranvier  the  formation  of  eleidin  is  followed  by  its  trans- 
formation into  keratin,  the  characteristic  substance  of 
horn,  nails,  claws,  etc.,  which  are,  as  we  shall  see,  in  the 
main  developments  of  the  stratum  lucidum. 

While  the  stratum  lucidum  is  constantly  receiving  acces- 
sions from  the  cells  of  the  stratum  granulosum  upon  its 
lower  or  inner  side,  it  is  as  constantlv  undergoing  mod- 
ifications on  its  upper  or  outer  surface ;  and  the  line  which 
marks  this  transformation  is  equall}'  well  defined  in  either 
case,  the  stratum  lucidum  remaining  like  the  stratum 
granulosum  of  nearly  constant  thickness  and  definite  lim- 
itations. Thej:ells  as  they  pass  from  its  outer  surface  be- 
come somewhat  swollen  and  more  loosely  disposed,  form- 
ing a  layer  in  which  the  outlines  of  the  individual  cells 
may  be  clearly  discerned  :  the  nuclei  have  disappeared  and 
all  traces  of  protoplasmic  structure.  This  layer  is  some- 
times designated  the  stratum  corneimi:  in  places  where 
there  are  no  hairs  upon  the  surfacp  it  can  be  divided  into 
two  layers,  the  lower,  or  stratum  epitrichium,  consisting 
of  thicker  cells,  more  loosely  disposed,  and  the  outer  or 


138  PART    II.      HISTOLOGICAL   ANATOMY. 

stratum  Stiuamosum  of  thin  closely  oppressed  scales, 
which  are  eventually  and  constantly  cast  oflP.  The  stra- 
tum lucidum  and  stratum  corneum  (with  the  subdivisions 
of  the  latter)  together  make  up  the  horny  layer. 

The  epidermis  varies  greatly  in  thickness  in  different 
parts  of  the  body:  it  may  be  no  more  than  a  tenth  of  a 
millimetre  in  thickness,  or  may  be  as  much  as  a  millimetre 
or  more  in  places  where  the  pressure  ai»d  friction  upon  the 
surface  is  greatest,  even  under  ordinary  circumstances : 
and  under  special  conditions  the  cells  of  the  stratum  squa- 
mosum of  the  palms  of  the  hand  or  the  soles  of  the  feet, 
instead  of  being  exfoliated,  may  become  impacted  to  form 
la^-'ers  two  or  three  millimetres  in  thickness.  The  external 
surface  shows  irregularities  which  in  some  measure  corres- 
pond to  the  conformation  of  the  corium  below,  but  as  a 
rule  differ  therefrom  by  their  less  extent :  the  surface  of  the 
corium  is,  as  we  shall  see,  covered  by  projections  known 
as  papillae,  into  the  spaces  between  which  the  stratum 
Malpighii  descends :  but  this  stratum  varies  correspond- 
ingly in  thickness  to  such  an  extent  that  the  papillary 
elevations  are  reduced  on  its  upper  surface  to  mere  un- 
dulations, to  which  the  remaining  strata  conform. 
The  deeper  lines  upon  the  surface  of  the  epidermis  corre- 
spond to  definite  folds  in  the  corium. 

Like  the  epidermis,  the  corium  can  be  divided  into  strata, 
which  are  not,  however,  so  clearly  defined  in  the  case  of 
the  latter  as  of  the  former,  consisting  as  they  do  in  modifica- 
tions of  a  fibrous  layer.    Upon  the  surface  the  fibres  are 


CHAPTER   Xm.      SKIN'   AND  APPENDAGES.  139 

fine  and  very  eloscly  felted,  forming  a  thin,  homogeneous 
and  almost  translucent  stratum,  which  may  be  distin- 
guished as  the  basement  membrane  of  the  epidermis.  Its 
surface  is  closely  beset  with  minute  projections,  which 
interlock  with  corresponding  irregularities  at  the  lower 
extremities  of  the  columnar  cells  which  lie  at  the  base  of 
the  stratum  Malpighii. 

Beneath  the  basement  membrane  above  referred  to  the 
corium  consists  of  a  felted  mass  of  rather  coarse  bundles 
of  white  fibrous  tissue,  reinforced  by  elastic  fibres  in  vary- 
ing quantity,  and  containing  in  some  localities  a  greater 
or  less  amount  of  smooth  muscular  fibres,  notably  in  con- 
nection with  the  hair-follicles.  The  outer  portion  is  denser, 
the  bundles  being  smaller  and  more  closely  felted  and 
h'ing  in  the  main  parallel  to  the  surface:  it  bears  the 
papillae  already-  referred  to  in  another  connection,  and  has 
therefore  been  variously  designated  from  its  structure  and 
conformation  as  the  dense  stratum  or  the  stratum  papil- 
lare:  below  it  passes  rather  abruptU',  but  with  no  well- 
defined  line  of  demarkation,  into  a  region  in  which  the 
bundles  are  coarser  and  less  numerous,  being  more  looseh' 
and  irregularly  disposed :  from  its  structure  this  is  known 
as  the  stratum  reticulare.  The  papillae  are  conical  or 
club-shaped  projections  upward  of  the  dense  layer:  in 
man)'  cases  they  are  more  or  less  subdivided  at  their  free 
extremities,  in  which  case  the}'  are  known  as  compound 
papillae:  they  are  most  abundant  in  the  regions  of  the 
skin  where  the  sense  of  touch  is  most  acute,  and  also  in  a 
modified  form  make  up  the  dermal  portion  of  the  nail- 
bed.     In  some  places,  notably  on  the  tips  of  the  fingers, 


140  PART    11.      HISTOLOGICAL   ANATOMY. 

they  are  arranged  in  single  or  double  rows  along  ridges  of 
the  corium,  forming  the  familiar  patterns  readily  seen  with 
the  naked  eye:  these  patterns  remain  constant  for  each 
digit  of  each  individual  throughout  life,  and  have  therefore 
been  used  as  marks  of  identification.  Where  best  devel- 
oped the  papillae  are  from  an  eighth  to  a  fourth  of  a  milli- 
metre in  height. 

The  connective  tissue  corpuscles  of  the  corium,  like  those 
of  other  fibrous  membranes,  are  small,  and  as  a  rule  com- 
pressed, their  long  axes  lying  parallel  to  the  direction  of  the 
bundles  with  which  they  are  associated :  leucocytes  are 
also  present,  as  are  pigment  cells  in  moderate  numbers. 

'  The  mcvshes  of  the  stratum  reticulare  not  unfrequently 
contain  clusters  of  fat-cells  of  varying  extent :  immedi- 
ately beneath,  in  most  portions  of  the  body,  we  come  to 
a  la^^er  of  larger  or  smaller  ovoid  or  polj^hedral  fat-lobules 
separated  by  a  coarse  meshwork  of  fibre  bundles.  This 
layer,  frequently  termed  the  panniculus  adiposus,  is  often 
spoken  of  as  subcutaneous :  it  is,  however,  in  many  cases 
no  more  clearly  marked  off  from  the  stratum  reticulare 
than  is  the  latter  from  the  stratum  papillare :  it  may  with 
good  reason  be  regarded  as  a  portion  of  the  skin,  and  as 
such  be  distinguished  as  the  stratum  adiposum:  whether 
it  shall  be  regarded  as  cutaneous  or  subcutaneous  is 
largely  a  matter  of  definition ;  the  fact  of  its  association 
with  the  other  structures  of  the  skin  as  a  part  of  the  tem- 
perature regulating  mechanism  of  the  body  is  important. 
Below  it  passes  into  the  loose  layer  of  subcutaneous  areo- 
lar tissue,  which,  save  in  a  very  few  localities,  intervenes 


CHAPTER     XIII.      SKIN    AND    A ITKNDACES.  14-1 

between  the  skin  and  the  structures  beneath,  permitting 
of  its  more  or  less  free  movement  upon  them. 

The  skin,  exclusive  of  the  stratum  adiposum,  varies  in 
thickness  from  h^lf  a  millimetre  to  two  or  three  milli- 
metres: and  may  even  occasionally  be  as  much  as  twice  the 
latter  quantity  in  thickness.  It  is  thickest  upon  the  shoul- 
ders and  back. 

The  arteries  which  pass  to  the  skin  branch  and  subdivide 
in  the  subcutaneous  tissue,  the  small  vessels  thus  formed 
proceeding  toward  the  surface  :  on  their  way  they  give  off 
twigs  which  supply  the  fat-lobes  above  mentioned,  and  the 
hair  follicles,  sweat  glands,  etc.,  presently  to  be  described. 
As  they  approach  thesurface  they  branch  and  anastomose, 
and  finally  break  up  into  a  meshwork  of  capillaries,  situ- 
ated j  ust  below  the  basement  membrane,  suppK'ing  the  pap- 
illae together  with  the  other  portions  of  the  dense  layer.  The 
capillaries  unite  to  form  a  superficial  venous  network,  the 
larger  veins  arising  therefrom  passing  to  the  deeper  por- 
tions of  the  skin  in  such  a  way  as  to  accompany  the  ar- 
teries in  large  measure. 

Lymphatics  arise  in  the  spaces  between  the  bundles  of 
fibres  which  make  up  the  dense  layer.  These  are  so  dis- 
posed as  to  form  a  lymphatic  network  just  below  the 
superficial  capillary  network  just  mentioned :  lymphatics 
have  also  been  demonstrated  in  some  of  the  larger  papil- 
lae. A  second  network  is  said  to  exist  in  the  deeper  por- 
tion of  the  corium,  the  two  communicating  freely  with 
each  other  and  with  the  subcutaneous  lymphatics. 


142  PART    11.      HISTOLOGICAL  ANATOMY, 

The  nerve  supply  of  the  skin  varies  greatly  in  different  pot' 
tions  of  the  bod\'.  Like  the  blood  vessels,  the  nerves  form 
plexuses  in  the  papillary  region,  the  meshes  immediately 
beneath  the  epidermis  becoming  ver^^  fine  and  close  :  from 
the  fibres  composing  them  fibrillae  are  sent  up  into  the 
stratum  Malpighii  in  the  manner  alread\^  described.  As 
stated  in  a  previous  chapter,  it  is  not  yet  certain  how 
these  fibrils  terminate. 

In  certain  (if  not  all)  portions  of  the  bod}',  more  or 
fewer  of  the  papillae  are  supplied  with  nerve  fibres  which 
there  terminate  in  the  so-called  tactile  corpuscles  of  Meiss- 
ner:  these  are  most  abundant  in  the  papillae  of  the  fingers 
and  toes.  Other  fibres  terminate  in  end  bulbs,  while  man}' 
others  are  distributed  to  the  hair  follicles  and  the  s-weat 
glands.  In  the  deeper  or  subcutaneous  region  fibres  are 
found  which  end  in  Pacinian  bodies. 

The  glands  of  the  skin  are  chiefly  of  two  kinds:  the 
sudoriparous  or  sweat  glands,  distributed  in  Yarjmg 
abundance  over  the  whole  surface  of  the  body ;  and  the 
sebaceous  glands,  found  chiefl}'  at  the  bases  of  the  hairs. 
In  certain  localities  specialized  glands  are  found  which  maj' 
be  regarded  as  modifications  of  one  or  the  other  of  these 
two  types. 

The  sudoriparous  glands  are  situated  in  the  reticular 
stratum  and  the  outer  portion  of  the  adipose  layer :  they 
are  spheroidal  bodies,  from  one-half  of  a  millimetre  to 
two  millimetres  in  diameter,  each  consisting  of  a  small 
tube  coiled  into  a  ball :  from  this  the  tube  proceeds  (as  the 
duct  of  the  gland)  with  slight  deviations  from  a  direct 


CHAPTER     XIII.      SKIN    AND    A  PrHNDAC.ES.  143 

course  through  the  coriuni,  at  whose  surface  it  becomes 
continuous  with  a  closely  coiled  spiral  opening  through 
the  epidermis  ;  the  whole  structure  consisting,  therefore,  of 
a  tubular  depression  of  the  surface,  whose  basement  mem- 
brane is  a  continuation  of  that  which  forms  elsewhere  the 
outer  limit  of  the  cprium,  while  its  lining  epithelium  is  a 
direct  continuation  of  the  epidermis. 

The  glandular  (or  the  greater  part  of  thecoiled  portion) 
is  considerably  larger  than  the  rest  of  the  tube,  being 
sixty  or  seventy  micra  in  diameter:  immediately  upon  the 
basement  membrane  in  this  region  is  found  a  simple  layer 
of  elongated  elements  resembling  smooth  muscular  fibres 
in  appearance,  and  commonly  regarded  as  such  ;  their  long 
axes  are  parallel  with  the  direction  of  the  tube.  Upon  this, 
and  surrounding  the  lumen  of  the  tube,  is  a  layer  of  col- 
umnar glandular  cells  about  fifteen  micra  in  diameter, 
which  are  frequently  pigmented.  The  duct  is  from  twenty 
to  thirty  micra  in  diameter:  it  consists  of  a  basement 
membrane  upon  which  is  seated  a  stratified  epithelium  of 
a  very  simple  order:  immediately  next  the  membrane  is  a 
layer  usually  two  cells  thick,  composed  of  polvhedral  cells ; 
while  upon  this  is  a  single  layer  ol  flattened  cells  often 
designated  the  cuticle.  On  reaching  the  epidermis  the 
basement  membrane  is,  of  course,  continued  into  that  of 
the  surrounding  region :  the  deeper  portion  ot  the  epithe- 
lium becomes  continuous  with  the  stratum  Malpighii :  the 
cuticular  layer  lines  the  passage  through  the  epidermis, 
which  is  from  the  first  over  twice  the  diameter  of  the 
lumen  of  the  duct,  and  in  the  stratum  corneum  flares  to 
form  a  trumpet-shaped  opening  at  the  surface. 


144  PART    II.      HISTOLO '.ICAL  ANATOMY. 

Each  sweat  gland  has  a  network  of  capillaries  which 
penetrates  the  coil,  following  the  interstitial  connective 
tissue ;  and  is  also  provided  with  a  proper  nerve  supply. 
The  number  of  the  glands  varies  in  different  portions  of 
the  bod}'  from  four  or  five  hundred  to  the  square  inch  (or 
more  than  a  millimetre  apart)  upon  the  back  of  the  neck 
and  trunk  to  five  or  six  time  as  many  (or  less  than  half  a 
millimetre  apart)  on  the  palm  of  the  hand  or  the  sole  of 
the  foot.  Those  of  the  armpits  are  quite  large,  as  are  those 
at  the  root  of  the  penis  in  the  male  and  on  the  labia 
majores  in  the  female.  About  the  anus  are  found  glands 
identical  with  the  sweat  glands  in  structure,  but  still 
larger  than  those  just  mentioned :  they  are  sometimes  dis- 
tinguished as  the  circumanal  glands.  In  the  larger  sudor- 
iparous glands  (and  sometimes  in  the  smaller)  branching 
of  the  coiled  portion  occasionally  occurs :  and  in  some 
cases  the  duct  is  bifurcated,  either  before  or  after  it  enters 
the  epidermis.  The  ceruminous  glands  of  the  external 
auditory  meatus  of  the  ear  resemble  the  sweat  glands 
in  their  structure,  though  differing  from  them  to  a  marked 
degree  in  the  nature  of  their  secretion. 

The  sebaceous  glands  occur,  as  has  been  stated,  at  the 
roots  of  the  hairs :  they  are  also  found  upon  the  labia 
minores  and  the  prepuce  and  occasionally  in  other  hairless 
localities.  Each  consists  of  a  short  duct  usually  opening 
into  the  outer  portion  of  a  hair  follicle  and  connecting  in- 
ternally with  the  adjacent  secreting  portion:  this  commonly 
consists  of  from  three  or  four  to  eighteen  or  twenty  sac- 
cules, or,  rarely,  a  single  spheroidal  sac.  The  basement 
membrane,  which  is  continuous  with  that  of  the  corium, 


CIIAPTHR    XIII.      SKIN    AM)   APPENDAGES.  14-') 

supports  a  conipouiul  layer  of  polyhedral  cells,  wliich  in 
the  glandular  portion  are  constantly  being  renewed  by  the 
division  of  the  cells  next  the  basement  membrane.  The 
cells  thus  cast  off  undergo  a  sort  of  fatty  degeneration, 
the  interior  of.  the  gland  thus  becoming  filled  with  a  semi- 
Huid  mass  consisting  of  oil  droplets,  fragments  of  broken- 
down  cells,  etc.,  which  is  discharged  upon  the  surface 
through  the  duct.  The  Meibomian  glands  of  the  eyelid 
are  to  be  regarded  as  enlarged  and  modified  sebaceous 
glands.  The  mammary  glands  are  also  to  be  regarded  as 
originally  derived  from  the  modification  of  bodies  essen- 
tiallv  similar  to  sebaceous  glands  and  are  properly  to  be 
considered  as  tegumentary  organs.  On  account  of  their 
size  and  complexity,  however,  their  structure  will  be  bet- 
ter understood  after  thediscussionof  the  general  structure 
of  glands  in  a  subsequent  chapter.  They  will,  therefore, 
be  discussed  later  in  connection  with  the  female  reproduc- 
tive organs. 

Both  sudoriparous  and  sebaceous  glands  are  formed  by 
solid  ingrowths  of  the  epidermis  which  in  the  case  of  the 
former  penetrate  the  corium,  the  cavity  being  for-med  in 
the  centre  of  the  club-shaped  mass  and  later  communicat- 
ing with  the  surface,  while  the  inner  end  becomes  coiled 
into  the  shape  characteristic  of  the  adult  structure.  In  a 
similar  manner  the  mass  of  cells  which  is  the  precursor  of 
the  sebaceous  gland  becomes  divided  into  lobules,  the  fatty 
transformation  of  the  central  cells  already  described  be- 
ginning in  them  and  advancing  along  the  axis  of  the  pedi- 
cle, which  thus  becomes  converted  into  the  duct. 


l-iG  PART    II.      HISTOLO    ICAL   ANATOMY. 

While  the  hairs  are  practicalh^  outgrowths  from  the 
skin  thej^  are  in  reality  derived  from  the  modification  of 
ingrowths  resembling  in  some  respects  those  which  give 
rise  to  the  sebaceous  and  sudoriparous  glands.  A  hair  is 
a  mass  of  epidermal  cells  which  cohere  together  to  form  a 
cylindrical  or  more  or  less  flattened  rod :  these  cells  are 
formed  upon  and  around  a  papilla  situated  at  the  bottom 
of  a  depression  of  the  corium  known  as  a  hair-foUicle, 
whose  sides  are  lined  with  a  specially  modified  layer  of  epi- 
dermal cells,  continuing  the  Malpighian  layer  of  the  epi- 
dermis and  the  stratum'  lucidum  to  the  region  around  the 
papilla  at  its  base.  The  portion  of  the  hair  which  pro- 
jects beyond  the  surface  is  termed  the  shaft:  that  which  is 
contained  in  the  follicle  is  known  as  the  root:  the  epider- 
mic portion  of  the  follicle  surrounding  the  root  is  com- 
monly called  the  root-sheath. 

The  fully  formed  hair,  whether  within  or  without  the 
follicle,  has  on  its  surface  a  layer  of  imbricated  scales,  with 
their  free  edges  directed  toward  the  outer  end  of  the  hair ; 
this  layer  is  known  as  the  cuticle:  on  the  form  and  ar- 
rangement of  the  free  margin  of  thecuticular  cells  depends 
the  pattern  of  the  superficial  markings  seen  on  the  hairs  of 
man  and  of  different  animals.  Immediately  beneath  the 
cuticle  lies  the  cortical  substance  of  the  hair:  this  is  com- 
posed of  greatly  elongated  fusiform  cells  in  which  traces 
of  the  nucleus  are  still  visible,  though  the  body  of  the  cell 
has  largely  imdergone  horny  transformation  :  these  cells 
are  so  closely  united  that  their  limits  are  not  ordinarily 
distinguishable,  the  cortex  appearing  to  be  made  up  of 
elongated  fibres ;  it  is  to  the  color  of  the  cortical  cells  that 


IokJx.IjU.^-'^-'^^^  " 


ClIArTKK     XIII.      SKIN    AND   AI'I'HNDAGIiS.  14-7 

the  color  of  the  hair  is  largely  tlue.  The  cortical  substance 
in  some  hairs  extends  clear  to  the  centre:  in  others  it  sur- 
rounds (as  its  name  imj)lies)  an  axial  mass  ofcuboidal  or 
polyhedral  cells,  the  medulla:  the  bodies  of  these  cells 
usually  contain  small  air-vesicles,  rendering  the  medulla 
white  by  reflected  light.  Such  air  s])aces  may  also  be 
present  in  the  elongated  cells  of  the  cortex. 

The  formation  of  the  hair  at  the  papilla  will  be  more 
clearly  understood  if  its  description  is  preceded  b\'  that  of 
the  follicle  with  whose  epidermis  it  is  continuous:  this,  like 
the  structure  of  which  it  is  a  modification,  consists  of  a 
fijjrous  or  dermal  portion  and  an  epithelial  or  epidermal  por- 
tion. The  dermal  portion  maybe  resolved  into  three  layers: 
of  these  the  outerfibrouslayerconsistsof  bundles  of  white 
fibrous  tissue  running  chiefly  in  the  direction  of  the  follicle 
and  containing  numerous  connective-tissue  corpuscles ;  it 
resembles  the  corium  in  most  respects,  save  in  the  absence  of 
elastic  fibres :  the  middle  layer,  sometimes  called  the 
muscular  layer,  consists  of  connective  tissue  whose  fibres 
run  transversely,  and  of  transversely-disposed  elongated 
cells  with  rod-shaped  nuclei  which  much  resemble  smooth 
muscular  fibres  and  have  been  described  as  such,  but  which 
may  perhaps  be  regarded  as  modified  connective-tissue 
corpuscles;  this  layer  extends  from  the  bottom  of  the  fol- 
licle to  the  point  where  the  sebaceous  gland  opens,  beyond 
which  it  is  wanting:  the  inner  or  hyaline  layer,  or  glassy 
membrane,  as  it  is  sometimes  termed,  is  thin,  homogene- 
ous and  transparent ;  it  corresponds  to  the  basement 
membrane  of  the  corium. 


148  PART    II.      HISTOLOGICAL   ANATOMY. 

The  epithelial  portion  of  the  follicle,  while  it  is  structur- 
ally free  from  the  hair  within,  is  generally  brought  awa\'- 
with  the  latter  if  it  be  pulled  out  of  the  skin  during  life: 
it  is  therefore  commonly  termed  the  root-sheath  of  the 
hair.  It  consists  of  two  layers,  comparable  in  a  general 
way  to  the  mucous  and  the  horny  layers  of  the  epidermis. 
The  outer  of  these,  or  outer  root-sheath,  is  much  thicker 
than  the  inner,  being  a  layer  several  cells  deep ;  of  these 
the  cells  next  the  hyaline  layer  (or  basement  membrane) 
are  columnar,  like  those  at  the  base  of  the  stratum  Mal- 
pighii ;  while  those  within  are  polyhedral  prickle  cells  like 
the  majority  of  those  in  the  stratum  just  mentioned. 

The  inner  layer  of  the  epithelium,  or  inner  root-sheath, 
may  be  divided  into  three  strata:  the  outer  of  these, 
known  as  Henle's  layer,  is  a  single  layer  of  flattened  cells 
of  a  horny  appearance,  in  which  nuclei  are  not  distinguish- 
able :  wnthin  this  is  Huxley's  layer,  composed  of  polyhe- 
dral cells  with  small  nuclei,  the  layer  being  two  or  three 
cells  deep:  this  is  lined  on  the  inside  by  the  cuticle  of  the 
root-sheath  a  single  layer  of  flattened  cells  which  are  im- 
bricated in  a  manner  similar  to  that  of  the  cells  of  the 
cuticle  of  the  hair,  but  in  the  opposite  direction,  the  free 
edges  of  the  cells  in  question  being  directed  toward  the 
bottom  of  the  follicle ;  as  a  consequence,  their  edges  inter- 
lock with  those  of  the  cells  on  the  surface  of  the  hair.  The 
name  of  inner  root-sheath  is  sometimes  restricted  to  the 
part  which  comprises  Henle's  and  Huxley's  layers,  the 
two  becoming  confluent  at  base  of  the  follicle ;  the  cuticle 
of  the  root-sheath  is  in  such  cases  described  as  third  layer 
thereof;  but  this  is  entirely  a  question  of  names:  no  satis- 


CHAPTHK    XIII.      SKIN    AM)   A  I'I'K.\I)A(iKS.  14-9 

factory  attem])t  has  been  made  to  identify  either  of  these 
layers  with  the  various  strata  of  the  skin  in  the  way  that 
the  outer  root-sheath  can  l)e  identified  with  the  stratum 
Malpighii. 

As  we  pass  to  the  base  of  the  follicle  the  outer  fibrous 
layer  is  continued  around  its  curving  extremity"  to  be  con- 
tinued into  the  hair  papilla,  a  large  club-shaped  papilla 
which  projects  upward  into  the  lower  end  of  the  follicle: 
the  middle  layer  becomes  thinner  and  terminates  near  the 
lower  end  of  the  follicle :  the  hyaline  layer,  now  resting  on 
the  fibrous  layer,  invests  the  surface  of  the  papilla  as  a 
basement  membrane.  The  whole  follicle  is  slightly  enlarged 
or  bulbous  at  the  base;  the  thickening  being  chiefl\'due  to 
an  enlargement  of  the  epithelial  portion  of  the  follicle  and 
of  the  lower  end  of  the  hair  itself  The  outer  root-sheath 
is  continued  downward  with  little  change,  the  columnar 
cells  at  its  base  passing  around  the  curve  of  the  valley 
which  surrounds  the  base  of  the  papilla  and  over  the  sur- 
face of  that  body,  acting  here  as  elsewhere  as  the  genera- 
tive layer  of  the  epithelium :  the  poU'hedral  cells  become 
confluent  with  those  which  form  the  layers  of  Huxle>^  and 
of  Henle,  these  latter  having  previously  become  merged 
into  one,  and  with  the  cells  of  the  cuticle  of  the  root-sheath. 
The  valley  at  the  base  of  the  papilla  is  thus  filled  with  a 
mass  of  newly  formed  cells,  which,  as  they  are  rapidlv 
multiplied,  are  pushed  off  as  a  cylindrical  mass,  the  hair, 
from  around  the  papilla.  The  medulla  of  the  hair  is  formed 
from  the  cells  developed  upon  the  upper  end  of  the  papilla 
itself,  and  is  in  a  certain  sense  the  continuation  of  the  col- 


150  PART    II.      HISTOLOGICAL  ANATOMY. 

umnar  layer  of  the  outer  root-sheath :  the  cortex  of  the 
hair  represents  the  polyhedral  layer  and  the  la^^ers  of  Hux- 
ley and  Henle ;  while  the  cuticle  of  the  hair  corresponds  to 
that  of  the  root-sheath. 

The  outer  fibrous  layer  of  the  hair  follicles  is  richly  sup- 
plied with  blood  vessels  and  nerves  :  some  fibres  of  the  lat- 
ter pass  to  the  outer  root-sheath,  where  they  appear  to 
terminate  among  the  epithelial  cells  in  a  manner  similar 
to  that  found  in  the  stratum  Malpighii  of  the  epidermis, 
chiefly  in  the  immediate  vicinity  of  the  sebaceous  glands. 
In  some  of  the  lower  animals  large  hairs,  chiefly  about  the 
face,  are  provided  with  special  forms  of  nerve  terminals : 
such  hairs  are  termed  tactile  hairs. 

The  hair  follicles  are  rarely  vertical  to  the  surface  of  the 
skin,  the  degree  of  their  obliquity  varying  in  different  local- 
ities and,  in  consequence,  the  position  of  the  hair  upon  the 
surface.  Many  hairs  have  small  bundles  of  smooth  mus- 
cular fibres  passing  from  a  point  on  the  papillary  layer  of 
the  corium  near  the  opening  of  the  follicle  and  on  the  side 
toward  which  the  hair  is  inclined,  to  be  inserted  in  the 
outer  fibrous  layer  of  the  follicle  near  the  bulb.  The  con- 
traction of  these  muscles,  known  as  the  arrectores  pili, 
tends  to  erect  the  hairs. 

Hairs  are  formed  as  solid  club-shaped  downgrowths  of 
the  stratum  Malpighii  of  the  epidermis,  which  meet  with 
specially  formed  papillae  around  which  the  hair-bulb  is 
moulded :  the  young  hair  is  developed  as  a  conical  mass 


CHAPTKK     XIII.      SKIN    AND    APrKNDAOES.  151 

above  the  pa])illa,  the  solid  epithelial  plug  first  formed 
undergoing  sebaceous  degeneration  in  its  centre  and  thus 
permitting  the  escape  of  the  hair:  its  lateral  portions  be- 
come the  root-sheath,  outgrowths  therefrom  giving  rise 
to  the  sebaceous  glands.  When  a  hair  ceases  to  grow,  the 
papilla  gradually  disappears  and  the  hair  finally  drops 
out  of  the  follicle:  this  may  or  may  not  have  been  pre- 
ceded by  the  formation  of  a  new  downgrowth  from  the 
bottom  of  the  follicle  and  the  development  of  a  new  pa- 
llia, thus  giving  rise  to  a  replacing  hair. 

The  nails,  like  hairs,  are  masses  of  epidermal  cells,  con- 
sisting chiefly  of  a  thickened  and  otherwise  modified  ex- 
tension of  the  stratum  lucidum.  Each  nail  can  be  regarded 
as  composed  of  three  portions :  the  free  margin,  in  which 
growth  has  entireh-  ceased,  the  nail-body,  which  consti- 
tutes its  greater  portion,  but  which  receives  but  slight  ad- 
ditions to  its  under  surface,  and  the  nail-root,  which  is  the 
region  of  greatest  increase.  The  body  of  the  nail  is  con- 
tinuous below  with  a  modification  of  the  stratum  Mal- 
pighii,  which  rests  upon  a  modified  portion  of  the  corium, 
called  the  nail-bed:  laterally  this  fibrous  layer  is  folded 
upward  to  form  the  lateral  nail-grooves,  and  posteriorly 
upward  and  forward  to  form  the  posterior  nail-groove ; 
the  lower  portion  of  which  is  termed  the  nail-matrix,  in- 
cluding the  whitish  curved  area  at  the  base  of  the  nail 
known  as  the  lunula. 

The  nail-bed  and  nail-matrix  are  continuations  of  the 
corium  which  has  become  highly  vascular  and  is  well  sup- 


152  PART    II,      HISTOLOGICAL  ANATOMY. 

plied  with  nerves :  the  papillae  upon  its  surface  are  simple 
and  closelv  crowded  tog-ether:  as  far  as  the  outer  marmn 
of  the  lunula  thev  show  no  definite  arrangement,  but 
throughput  the  nail-bed  proper  are  arranged  in  longitud- 
inal row^s,  their  extremities  inclining  toward  the  free  end 
of  the  nail :  the^-are  so  closeh'set  in  the  rows  as  to  appear 
to  be  confluent  in  ridges,  which  are  sometimes  said  to  re- 
place them.  Below,  the  nail-bed  is  connected  with  the  dis- 
tal extremit}'  of  the  last  phalangeal  bone  b\'  numerous 
strong  bands  of  fibrous  tissue:  as  it  passes  around  the  mar- 
gin of  the  nail  to  enter  the  walls  of  the  nail-grooves,  it 
assumes  the  structure  commonh^  characteristic  of  it. 

The  stratum  Malpighii  is  by  some  histologists  defined 
as  a  part  of  the  nail-bed  :  it  is  a  question  of  names  merely, 
but  it  is  perhaps  better  on  the  whole  to  divide  the  two  re- 
gions by  the  natural  boundary  between  dermal  and  epi- 
dermal structures.  The  columnar  cells  of  this  stratum 
are  close]}'-  packed  together,  and  multiply  rapidly,  partic- 
ularh'-  in  the  region  of  the  matrix:  very  few  polyhedral 
cells  are  to  be  seen,  the  newly  formed  cells  passing  over 
rapidly  into  the  substance  of  the  nail  without  an  inter- 
vening stratum  granulosum.  Like  the  stratum  lucidum, 
the  bod}^  of  the  nail  consists  of  flattened  horny  cells,  in 
which  traces  of  nuclei  can  be  seen  after  dissociation.  Dur- 
ing foetal  life  the  nail  is  invested  by  the  stratum  epitrich- 
ium,  traces  of  which  overlie  its  margins  at  birth. 


CIIAPTHR  XIY.      MOl^TH  AND  CONTENTS.  153 


CHAPTER  XIV. 
THE  MOUTH  AND  ITS  CONTENTS. 


The  mouth  is  formed  b}^  an  ingrowth  from  the  surface 
of  the  head  ;  its  lining  is  therefore  epiblastic  in  origin  and 
directl\'  continuous  with  the  epidermis:  the  cavity  of  the 
mouth  does  not  at  first  communicate  with  that  of  the  phar- 
3'^nx,  but  the  two  are  connected  later  by  the  perforation  of 
their  common  wall  at  the  fauces.  Within  the  cavity  of  the 
mouth  are  found  the  jaw  arches,  covered  by  the  gums  and 
bearing  the  teeth,  and  the  tongue,  which  rises  from  its 
floor;  and  in  the  so-called  mucous  membrane  which  lines 
it  throughout  are  found  the  labial,  buccal,  palatal  and  lin- 
gual glands :  while  other  and  larger  glands  more  remoteh- 
situated  discharge  their  secretion  into  the  cavity  of  the 
mouth  b}'^  means  of  ducts. 

As  we  pass  from  the  skin  upon  the  outer  surface  of  the 
lip  to  the  mucous  membrane  which  is  found  upon  its  inner 
surface  marked  changes  are  to  be  noted  in  both  the  epithe- 
lial and  the  fibrous  layer.  Near  the  margin  of  the  lip  the 
hair  follicles  are  wanting,  though  sebaceous  glands  are 
present ;  the  derma  becomes  thinner  and  highly  vascular ; 
while  the  epidermis  becomes  much  more  transparent,  al- 
lowing the  red  color  of  the  blood  in  the  dermal  capillaries 
to  shine  through.  As  the  transition  is  made  from  a  sur- 
face constantly  dr}-  from  exposure  to  the  air  to  one  con- 


154  PART    11.      HISTOLOGICAL  ANATOMY. 

stantly  moist,  the  stratification  of  the  egithelium  becomes 
less  distinct:  and  on  the  inside  of  the  mouth  is  found  a 
la^^er  of  protoplasmic  cells  corresponding  to  the  stratum 
Malpighii  passing  by  insensible  gradations  into  a  layer  of 
hornj^  cells,  flattened,  and  with  small  nudei,  that  are  con- 
stantly being  exfoliated,  corresponding  to  the  stratum  cor- 
neum :  all  trace  of  the  inter veniiig  stratum  granulqsum 
and  stratum  lucidum  disappearing  The  cells  at  the  base 
of  the  layer  are  columnar,  like  those  in  the  correspond- 
ing portion  of  the  epidermis,  and,  like  them,  are  con- 
stantly forming  new  cells  to  replace  those  lost  from  the 
outer  surface. 

Underneath  the  epithelium  is  the  fibrous  J[ayer,  corres- 
ponding to  the  corium,  to  w^hich  the  name  of  mucosa  or 
mucous  membrane  is  sometime  restricted :  it  is  in  most 
cases  thinner  than  the  corium,  but  bears  upon  its  surface 
numerous  papillae :  below  it  breaks  up  into  looser  connec- 
tive tissue  as  a  rule,  the  meshes  being  occupied  by  the 
glands  of  the  mucosa  and  by  fat  lobules :  to  bhis  looser 
subjacent  tissue  the  names  of  submucosa  is  applied:  in 
the  region  of  the  fauces,  the  soft  palate,  and  the  uvula  ad- 
enoid tissue  is  present  in  great  abundance  in  the  raucous 
membrane ;  a  feature  never  found  in  connection  with  the 
corium  of  the  skin. 

Where  the  mucous  membrane  invests  the  hard  palate, 
and  where  it  passes  over  the  arch  of  either  jaw  to  form 
the  gums,  the  fibrous  layer  becomes  firm  and  dense,  glands 
and  fat  lobules  alike  being  absent  or  very  sparingly  pres- 
ent, and  the  deeper  portion  becomes  directly  continuous 
with  the  periosteum  of  the  subjacent  bone.  The  papillae 
and  the  investing  epidermis  of  the  dorsal  surface  of  the 


CHAPTKK    XTV.      MOITII    AM)   CONTKNTS.  155 

tongue  undergo  special   modifications  best   described   in 
connection  with  that  organ. 

The  term  mucosa,  or  its  equivalent,  mucous  membrane, 
above  applied  to  the  lining  of  the  mouth,  is  also  used 
to  designatethe  lining  of  all  those  cavities  which  communi- 
cate directly  or  indirectly  with  the  outside  of  the  body 
(with  the  exception  of  the  abdominal  cavity  of  the  female). 
It  thus  includes  the  investment  of  the  nasal  passages  and 
the  associated  sinuses,  the  respirator\'  tract,  the  oral  cav- 
ity and  the  alimentary  canal,  the  urinogenital  tract,  and 
the  middle  ear  with  the  Eustachian  tube.  B\'  some  histol- 
ogiststhe  term  is  even  applied  to  the  lining  of  the  hair  folli- 
cles and  the  dermal  glands.  Excluding  the  latter,  it  may  in 
general  terms  be  described  as  consisting,  like  the  skin,  and 
as  seen  in  the  lining  of  the  mouth,  of  an  epithelial  and  a 
skeletal  portion.  The  epithelium  of  the  mouth  and  of  the 
nasal  passages  is  epiblastic  in  its  origin  :  that  of  the  kid- 
neys and  genital  glands,  and  of  their  proper  ducts,  meso- 
blastic :  that  of  all  the  rest  of  the  surfaces  named  hypo- 
blastic.  It  is  to  the  regions  lined  with  h3'poblastic  epithe- 
lium that  the  term  mucosa  is  chiefly  applied. 

The  epithelium  of  a  mucous  surface  ma\'  be  simple  and 
flattened,  polyhedral,  or  columnar;  or  transitional;  or 
stratified.  The  skeletal  layer  consists  usnallv  of  a  base- 
ment membrane  (sometimes  termed  a membrana  propria,) 
which  may  be  either  a  homogeneous  layer  of  closely  felted 
fibres,  or  an  endothelioid  layer  of  connective  tissue  cor- 
puscles ;  beneath  this  is  the  felted  fibrous  layer  termed  va- 
riously the  corium,  the  stroma,  or  the  tunica  propria,  or, 
by  some,   the  mucosti  in  a  limited  sense.     It  consists  of  a 


156  PART    II.      HISTOLOGICAL  ANATOMY. 

thicker  or  thinner  layer  of  rather  loosely  felted  bundles  of 
white  fibres,  rich  in  blood  vessels  and  in  some  cases  having 
its  surface  raised  into  papillae.  Elastic  fibres  are  some- 
times present  in  such  great  numbers  as  to  form  a  definite 
elastic  layer,  and  at  other  times  are  almost  entirely  want- 
ing. Adenoid  tissue  may  be  present  in  varying  quantity, 
sometimes  forming  definite  nodules  or  clusters  of  nodules. 
Smooth  muscular  fibres  may  also  be  present,  forming  a 
stratum  in  the  deeper  part  of  the  membrane  one  or  more 
layers  in  thickness  and  known  as  the  muscularis  mucosae. 
In  exceptional  cases,  such  as  that  above  described  in  con- 
nection with  the  hard  palate  and  the  jaw^  arches,  the  mu- 
cosa is  firmh'  united  to  the  subjacent  structures :  as  a  rule 
the  deeper  portion  passes  over  into  a  la^^er  of  loose  areolar 
tissue  knowm  as  the  submucosa,  thus  permitting  of  the 
free  movement  of  the  mucosa  on  the  structures  beneath. 

Where  the  epithelium  of  a  mucous  surface  is  columnar  a 
varying  proportion  of  the  elements  have  the  form  of  gob- 
let cells,  as  described  in  a  previous  chapter :  these  are  per- 
haps the  simplest  form  of  special  secreting  organs  found 
in  the  human  body ;  and  are  sometimes  spoken  of  as  uni- 
cellular glands :  the  term  gland  being  often  loosely  em- 
ployed to  designate  any  secreting  organ  and,  indeed,  some 
organs  that  are  not  at  all  secretory  in  function. 

Increase  of  secreting  surface  is  obtained  by  ingrowths 
of  the  epithelium  which  penetrate  the  fibrous  layer  to  a 
greater  or  less  extent,  in  a  manner  similar  to  that  de- 
scribed in  connection  with  the  skin ;  like  those  referred  to 
these  ingrowths  may  be  either  tubular  or  saccular.  Where 


CHAPTKR    XIY.      MOl  TH    AND   CONTENTS.  157 

such  ingrowths  are  not  sub-divided  internally  they  are 
commonly  called  simple  tubular  or  simple  saccular 
glands,  as  the  case  may  be.  A  distinction  of  importance, 
both  structural  and  physiological,  ought,  however,  to  be 
noted  in  this  connection.  Some  such  simple  secreting  or- 
gans are  lined  with  cells  that  are  similar  in  form  and  alike 
secretory  in  function  throughout  their  whole  extent : 
while  in  others  the  work  of  secretion  is  restricted  to 
specialized  cells  in  the  epithelium  of  the  deeper  portions, 
that  which  is  found  upon  the  portion  next  the  surface 
having  lost  its  secretorj'  activity'  and  become  modified  to 
form  the  lining  of  a  conducting  tube  through  which  the 
secretion  of  the  deeper  portion  is  discharged.  This  dis- 
tinction can  be  kept  in  view  if  we  always  apph'  to  the 
structure  which  secretes  throughout  its  whole  extent  the 
name  of  a  follicle,  and  define  a  gland  as  a  secreting 
organ  provided  with  a  duct. 

Glands,  as  thus  defined,  may  be  either  tubular  or  sac- 
cular, and  either  simple  or,  by  the  subdivision  of  the  se- 
creting portion,  multiple  or  compound:  the  secreting  por- 
of  a  simple  gland  is  called  the  fundus:  those  of  a  com- 
pound gland  are  called  acini  or  alveoli.  Where  the  final 
divisions  of  a  compound  gland  are  saccular  in  form,  the 
gland  is  frequently  designated  as  acinous  or  racemose ; 
where  they  are  elongated,  the  gland  is  called  compound 
tubular;  and  where  both  forms  of  alveolus  are  present  the 
term  acino-tubular  is  applied.  B\'  many  histologists, 
however,  the  distinctions  which  these  terms  imply  are  re- 
garded as  of  questionable  value. 

Follicles,  simple  glands,  and  the  smaller  compound  glands 


158  PART   II.      HISTOLOGICAL   ANATOMY. 

rareh'  penetrate  deeper  than  the  submucous  or  subcutane- 
ous tissue  of  the  membrane  Irom  which  they  are  derived. 
Large  compound  glands,  on  the  contrar3^  are  usually  sit- 
uated at  some  distance  from  the  surface  where  their  secre- 
tions are  discharged.  Glands  which  are  thus  situated  out- 
side of  the  organs  to  w^ich  their  ducts  lead  are  called  ex- 
trinsic, the  term  intrinsic  being  applied  to  those  contained 
Avithin  the  organs  in  question. 

With  the  exception  of  the  tegumentary  glands  discussed 
or  mentioned  in  the  preceding  chapter,  all  the  glands  of 
the  body  open  upon  mucous  surfaces ;  and  with  the  far- 
ther exception  of  the  kidne3'-s  and  the  genital  glands  are 
derived  from  the  modification  of  a  mucous  membrane. 
The  secretions  discharged  b}^  them  have  always  one  or 
both  of  two  functions :  to  lubricate  and  preserve  the  sur_ 
face  in  question,  which  is  a  general  function,  or  to  act  as 
ferments  upon  food  stuffs  taken  into  the  body,  which  is  a 
special  function  restricted  to  the  alimentary  tract.  Two 
principal  types  of  glands  are  therefore  distinguishable 
both  structurally  and  functionally.  Those  of  the  first  sort 
are  known  as  mucous  glands;  those  of  the  second  (from 
the  more  water}^  character  of  their  secretions,  but  less 
properly)  as  serous  glands.  In  the  case  of  some  com- 
pound glands  some  of  the  alveoli  are  of  the  mucous  and 
some  of  the  serous  t3^pe:  such  are  designated  mixed, 
glands. 

In  glands  of  the  mucous  type  the  acini  or  alveoli  are 
lined  with  polj^hedral  cells  which  do  not  extend  quite  to 
the  centre  of  the  cavity,  thus  leaving  a  well  defined  central 


CHAPTER    XIV.      MOl'TH    AND   CONTEXTS.  '[~>9 

opening  or  lumen:  the  free  extremity  of  the  eell  is  trans- 
parent and  does  not  stain  readily  with  most  reagents, 
while  the  ])rotoplasm  and  the  somewhat  flattened  nu- 
cleus which  it  surrounds  are  crowded  down  to  the  base* 
of  the  cell;  ijn  other  words,  the  general  appearance  is  like 
that  previously  described  as  characteristic  of  goblet  cells, 
which,  it  should  be  recalled,  are  mucigenous  in  function. 
In  addition,  there  may  in  some  cases  be  seen  between  the 
glandularcells  and  the  basement  membranecrescent  shai)ed 
groups  of  granular  cells  which  stain  deeply :  these  were 
described  independently  by  two  observers,  who  gave  them 
names  associated  with  their  form ;  they  are  therefore 
called  the  crescents  of  Gianuzzi,  or  the  demilunes  of 
Heidenhain:  their  nature  and  functions  are  not  yet  fully 
understood:  the  constituent  cells  (sometimes  solitary )  are 
called  marginal  cells. 

In  glands  of  the  serous  type  the  secreting  epithelium 
consists  of  polyhedral  cells  which  when  at  rest  extend  clear 
to  the  centre  of  the  alveolus:  a  distinct  lumen  cannot, 
therefore,  be  recognized.  The  whole  body  of  the  cell  is 
granular,  the  substance  which  is  to  become  the  character- 
istic secretion  being  thus  stored  up  in  the  protoplasm : 
the  nucleus  is  spherical,  and  situated  near  thecentre  of  the 
mass ;  and  the  whole  cell  stains  readily.  No  trace  of  any- 
thing like  the  crescentic  cell-masses  above  mentioned  or  of 
marginal  cells  is  to  be  found  in  any  serous  gland. 

The  appearances  above  described  are  those  seen  in  sec- 
tions of  glands  previousK'hardenedby  reagents,  and  taken 


160  PART    II.      HISTOLOGICAL   ANATOMY. 

trom  organs  which  had  not  been  actively  stimulated  im- 
mediately before  the  preparation  was  made.  If  a  piece  of 
the  fresh  gland  be  examined  in  blood  serum  the  mucigen 
is  seen  in  the  case  of  the  mucigenous  cells  to  be  present  in 
the  form  of  very  large  granules :  while  the  ferment  secre- 
ting cells  are  so  swollen  that  not  only  the  lumen  of  the 
acinus  but  also  the  outlines  of  the  cells  are  obliterated  : 
otherwise  the  appearances  are  much  as  described  above. 
As  the  result  of  prolonged  stimulation  the  mucigenous 
cells  discharge  their  secretion,  and  the  nuclei  approach  a 
central  position :  while  the  cells  of  the  serous  type  become 
smaller  after  discharge,  and  a  distinct  lumen  becomes  vis- 
ible in  the  acinus.  The  two  types  of  alveoli,  become 
therefore  much  more  nearly  alike,  though  not  so  much  so 
as  to  prevent  their  distinct  recognition. 

It  is  -in  the  mouth  that  we  first  meet  with  a  mucosa ; 
and  its  discussion  has  therefore  been  deferred  until  after  a 
description  of  the  lining  membrane  of  that  cavity.  The 
glands  of  the  mouth  also  furnish  usw^ith  examples  of  both 
mucous  and  serous,  and  both  intrinsic  and  extrinsic 
glands. 

The  intrinsic  glands  of  the  mouth  are  the  labial,  buc- 
cal, palatal  (including  those  of  the  uvula),  and  lingual. 
These  are  all  racemose  glands  situated  in  the  submucosa, 
with  the  exception  of  the  lingual,  which  lie  between  the 
muscular  bundles  of  the  tongue.  They  are  all  of  the  mu- 
cous t3^pe  save  those  on  the  posterior  part  of  the  tongue, 
which  are  serous.  Their  secretion  contributes  to  form  the 
saliva  of  the  mouth. 


CHAPTER    XIV.      MOUTir    A.NT)   CONTENTS.  101 

The  extrinsic  glands  of  the  mouth  are  those  commonh' 
referred  to  under  the  name  of  the  salivary  glands:  they 
are  dcslpiatcd,  from  their  positions  as  tlic  sublingual, 
the  submaxillary,  and  the  parotid  glands:  the  first- 
named  are  doubtfully  to  be  called  extrinsic,  on  account  of 
their  structure  and  position,  since  they  consist  in  each  in- 
stance of  a  group  of  small  glands  opening  by  several 
ducts,  and  are  situated  just  beneath  the  mucous  membrane 
on  either  side  of  the  base  of  the  frenum  of  the  tongue. 
The  nature  of  the  salivary  glands  differs  in  different  mam- 
mals: in  man  the  sublingual  and  submaxillary  are  mixed 
glands,  and  the  parotid  is  of  the  serous  type. 

The  acini  or  alveoli  of  one  of  the  larger  or  extrinsic 
salivary  glands  vary  greatl^^  in  form  from  flask-shaped 
sacs  to  wavy,  contorted,  or  even  branched  tubules:  the 
basement  membrane  is  reticulated  and  the  epithelium 
varies  with  the  type  of  gland  in  question.  Each  alveolus 
leads  into  a  tubule  smaller  than  itself  known  variously  as 
the  ductule,  the  connecting  or  intermediate  tubule  or 
the  intercalary  duct:  the  basement  membrane  of  the  duc- 
tule is  continuous  with  that  of  the  alveolus;  its  epithelium 
consists  of  a  sjngle  layer  of  flattened  cells.  The  ductules 
of  a  number  of  alveoli  enter  a  common  tube  known  as  an 
intralobular  duct  or  salivary  tube  of  Pflueger,  the 
ajveoli  and  ductules  with  the  duct  just  mentioned  together 
making  up  a  lobule.  An  intralobular  duct  consists  of  a 
basement  membrane  continuous  with  that  of  the  ductules, 
and  a  single  la3^er  of  columnar  epithelial  cells  :  the  latter 
have  spherical  nuclei  situated  near  the  centre  of  the  cell ; 


h^UJJrOJr  di^cli  r  (^J  ducJr 


162  PART   II.      HISTOLOGICAL   ANATOMY. 

the  outer  extremit}^  of  the  cell  is  finely  granular,  while 
that  next  the  basement  membrane  is  vertically  striated. 
The  basement  membrane  of  the  alveoli,  the  ductules  and 
the  intralobular  duct  alike  rest  upon  the  interstitial  con- 
nective tissue,  which  contains  rich  networks  of  capillaries 
surrounding  the  alveoli. 

The  intralobular  ducts  lead  into  larger  conducting  tubes 
known  as  interlobular  ducts,  around  which  the  lobules  are 
aggregated  into  lobes,  their  boundaries  within  the  lobe 
being  marked  by  septa  of  fibrous  tissue.  The  interlobular 
lead  into  the  interlobar  ducts,  which  finally  enter  the 
chief  duct  of  the  gland.  The  larger  ducts  have  beneath 
the  basement  membrane  a  definite  fibrous  layer  which  in 
some  cases  contains  smooth  niuscular  fibres :  the  epithe- 
lium is  columnar  and  simple,  save  in  the  largest  ducts,  in 
which  a  layer  of  polyhedral  cells  lies  betw^een  the  colum- 
nar cells  and  the  basement  membrane. 

The  interstitial  connective  tissue  which  fills  the  spaces 
between  the  alveoli  passes  into  thin  laminae  of  fibrous  tis- 
sue which  separate  the  lobules :  these  septa  are  again  con- 
tinuous with  stouter  structures  of  the  same  nature  which 
lie  between  the  lobes  of  the  gland.  The  interlobar  septa 
are  continuous  internally  with  the  stroma  of  connective 
tissue  which  immediatelj^  surrounds  the  proper  fibrous 
tunic  of  the  chief  duct  and  its  principal  subdivisions,  and 
externalh^  with  a  membranous  layer  which  surrounds  the 
whole  gland  and  is  known  as  its  capsule.  The  place 
where  the  chief  duct  leaves  the  interior  of  the  gland  is 
known  as  the  hilum:  the  nerves,  arteries,  veins,  and 
lymphatics  of  the  gland  also  enter  or  leave  at  this  point, 


CHAPTER    XIV.      MOITH    AND   CONTENTS.  1  Orj 

the  capsule  here  becomino^  continuous  with  the  stroma  of 
fibrous  tissue  (above  mentioned  as  surrounding  the  duct 
and  its  branches)  in  which  they  He,  and  in  which  occa- 
sional small  nervous  ganglia  may  be  found. 

The  saliva  contained  in  the  mouth  is  a  mixture  of  the 
secretions  of  the  various  intrinsic  and  extrinsic  glands. 
With  the  nature  and  properties  of  the  fluid  itself  we  are  not 
here  concerned :  it  constantly  contains,  however,  certain 
histological  elements  which  may  with  propriety  be  men- 
tioned in  this  connection.  The  most  abundant  of  these 
are  the  scpiamous  cells  which  are  constantly  being  exfol- 
iated from  the  surface  of  the  stratified  epithelium  of  the 
mouth  :  the}' occur  singlj^and  in  patches;  and  when  found 
in  the  latter  form  the  overlapping  of  their  bevelled  edges 
can  be  plainU'seen:  the  nuclei  arc  small  and  flattened,  and 
stain  readih'.  Less  numerous,  but  quite  abundant  in  the 
saliva  from  the  back  part  of  the  mouth  are  the  so-called 
salivary  corpuscles,  spheroidal  bodies  but  little  larger 
than  colorless  blood  corpuscles,  each  containing  one  or 
two  spheroidal  nuclei  and  numerous  minute  granules  which 
exhibit  a  constant  dancing  movement  within  the  interior 
of  the  cell.  The  salivary  corpuscles  are  really  modified 
leucocj'tes  that  have  escaped  into  the  oral  cavity  from  the 
tonsils  or  similar  adjacent  structures,  and  have  become 
swollen  by  the  imbibition  of  the  water}'  saliva;  the  gran- 
ules of  the  protoplasm  being  suspended  in  the  imbibed 
fluid  and  exhibiting  Brownian  motion  in  consequence. 

The  mouth  contains  certain  organs  by  means  of  which 


164  PART    II.      HISTOLO  UCAL   ANATOMY. 

the  food  is  masticated  and  prepared  for  swallowing  by  the 
thorough  admixture  of  the  saliva.  These  are  the  teeth 
and  the  tongue.  A  tooth  is  a  vertical]}^  elongated  mass  of 
the  tissue  mentioned  in  a  previous  chapter  by  the  name  of 
dentine,  having  a  solid  free  portion,  the  crown,  project- 
ing above  the  gum  and  covered  with  a  layer  of  enamel: 
and  a  hollow  portion,  the  fang,  imbedded  in  a  socket  in 
the  jaw  known  as  an  alveolus;  the  outer  surface  of  the 
fang  is  covered  with  a  layer  of  cementum,  and  the  inter- 
nal cavit}^  occupied  by  the  pulp.  The  slightly  constricted 
region  where  the  crown  and  the  fang  meet  and  the  tooth 
pierces  the  gum  is  called  the  neck  of  the  tooth. 

The  pulp  which  fills  the  central  cavity  of  the  tooth  is  a 
mass  of  connective  tissue  approaching  in  character  more 
nearly  to  the  gelatinous  tissue  of  the  embryo  than  any 
other  structure  in  the  adult  human  body,  its  soft  and  spar- 
ingly fibrillated  matrix  containing  numerous  branching 
corpuscles  whose  processes  are  connected.  It  contains  a 
rich  network  of  bloodvessels,  and  a  small  bundle  of  nerve 
fibres,  entering  its  substance  through  the  minute  canal  at 
the  tip  of  the  fang.  Toward  its  surface  the  corpuscles 
rapidly  increase  in  number  and  in  size,  forming  a  superfi- 
cial layer  of  crowded  cells  much  resembling  a  columnar 
epithelium :  these  are  the  odontoblasts ;  they  are  in  all 
probability  directly  associated  with  the  formation  of  den- 
tine. 

Like  osseous  tissue,  dentine  is  characterized  by  a  lamel- 
lated  and  calcified  matrix  traversed  by  canaliculi :  it 
differs  essentially  from  the  substance  of  bone,  however,  in 
the  mode  of  calcification  and  the  definiteness  of  the  lamel- 


CHAPTKR    XIY.      MOl'TH    AND   CONTEXTS.  165 

lae,  and  particularly  in  that  the  characteristic  corpuscles 
associated  with  its  formation,  the  odontoblasts,  do  not, 
like  the  osteoblasts  of  bone  tissue,  become  imbedded  be- 
t\veen  the  lamellae,  but  remain  situated  upon  the  inner 
surface  of  the  mass,  their  long  and  sparingly  branched 
processes,  the  fibres  of  Tomes,  occupying  the  canals,  called 
dentinal  tubules,  which  traverse  the  dentinal  lamellae. 
Calcification  takes  place,  at  least  in  the  outer  portion  of 
the  mass,  by  the  deposition  of  globular  nodules  of  lime 
salts;  these  at  first  do  not  occupy  the  whole  of  the  matrix, 
leaving  numerous  irregularly  stellate  spaces,  the  inter- 
globular spaces,  the  outer  layer  in  which  they  abound 
being  known  as  the  granular  layer.  The  interglobular 
spaces  communicate  outwardly  with  the  surface  of  the 
dentine  and  inwardK'  with  the  dentinal  tubules :  corpus- 
cles have  been  described  as  contained  in  them  and  commu- 
nicating with  the  extremities  of  the  fibres  of  Tomes. 
Deeper  the  calcification  becomes  more  nearh'  continuous  ; 
at  certain  intervals  it  is,  however,  incomplete,  irregular 
lines  in  a  general  way  parallel  to  the  surface  being  seen  : 
these,  known  as  the  incremental  lines  of  Salter,  may  be 
regarded  as  marking  the  boundaries  of  the  lamellae.  The 
dentinal  tubules  are  lined  b}-  a  condensation  of  the  fibres 
of  the  matrix  sometimes  described  as  a  distinct  membrane; 
they  have  a  wavy  or  spiral  course  across  the  lamellae  and 
give  off  branches  occasionally  at  acute  angles  which  pur- 
sue a  similar  course;  their  waviness  gives  rise  to  a  striated 
appearance  when  a  tooth  is  seen  in  longitudinal  section 
with  the  naked  eye,  or  a  low  pow-er  of  the  microscope : 
the  alternating  dark  bands  seen  are  know^n  as  the  lines  of 


166  PART    II.      HISTOLOGICAL  ANATOMY, 

Schreger.  The  general  direction  of  the  tubules  is  in  the 
fang  at  right  angles  to  the  lamellae ;  in  the  crown  they 
pass  obliquely  upward. 

The  enamel  consists  of  a  layer  of  elongated  calcified 
prisms,  usually  hexagonal  in  cross  section,  set  in  a  general 
way  vertical  to  the  surface  of  the  dentine  of  the  crown  of 
the  tooth ;  they  are  alternated  lighter  and  darker  through- 
out their  extent,  giving  to  the  enamel  as  a  whole  a  banded 
appearance.  In  addition,  occasional  dark  brov\rn  lines 
may  be  seen  crossing  the  enamel  columns  parallel-wise  to 
the  surface;  these,  known  as  the  stripes  of  Retzius,  may 
indicate  lines  of  growth.  Here  and  there  vertical  spaces 
may  be  seen  slightly  separating  the  enamel  columns  near 
the  dentine,  with  which  the  interglobular  spaces  of  the 
latter  possibly  communicate.  At  the  time  of  irruption 
the  surface  of  the  enamel  is  covered  with  a  thin  cuti- 
cular  layer  know^n  as  Nasmyth's  membrane;  it  is  rapidly 
worn  away  as  soon  as  the  teeth  are  put  to  active  use. 

The  cementum,  cement-substance,  or  crusta  petrosa,  as 
it  is  variously  termed,  investing  the  surface  of  the  fang,  is 
composed  of  tissue  essentially  similar  in  structure  to  that 
of  dense  bone :  the  lamellae  are  few  and  irregularly  ar- 
ranged, the  lacunae  varying  in  size  and  form ;  the  canali- 
culi  of  the  latter  are  said  to  communicate  with  the  inter- 
globular spaces  of  the  adjacent  dentine:  there  are  no 
clearly  defined  Haversian  systems.  The  layer  is  formed  by 
the  so-called  periodontal  membrane,  which  is  practically 
continuous  outwardly  with  the  periosteum  lining  the 
alveolus  which  contains  the  tooth. 


CHAPTER    XIV.      MOITII    AND   CONTENTS.  107 

The  enamel  is  epithelial  in  its  origin,  being  derived  from 
the  calcification  of  a  layer  of  cells  or  formed  by  their  secre- 
tion. The  dentine  and  ccmcntum  are  true  skeletal  struc- 
tures. The  margin  of  the  foetal  jaw  early  shows  a  thick- 
ening of  the  stratified  epithelium  which  grov/s  downward 
into  a  groove  into  the  mesoblastic  tissue  beneath,  the 
dental  groove:  the  curved  rod  of  epithelial  cells  thus 
formed  is  known  as  the  primary  enamel  germ.  At  regu- 
lar intervals  along  the  under  side  (jf  this  rod  further  prolif- 
erations of  the  e])ithelial  cells  occur,  with  localized  down- 
growths,  the  special  enamel  germs,  at  first  flask-shaped. 
At  the  same  time  a  condensation  in  the  mesoblastic  tissue 
beneath  each  gives  rise  to  a  conical  dental  papilla  which 
grows  upward  to  meet  the  epithelial  downgrowth.  The  lat- 
ter grows  down  about  the  papilla,  thus  becoming  converted 
into  a  cap-like  mass,  the  enamel  organ,  connected  with 
the  epithelial  ridge  above  b}-  a  slender  stalk:  the  stalk  is 
attached  to  one  side  of  the  enamel  organ,  owing  to  the 
mode  of  growth  of  the  latter.  A  condensation  of  the 
mesoblastic  tissues  surrounding  the  newly  formed  enamel 
organ  and  papilla  gives  rise  to  a  membrane  which 
soon  becomes  rich  in  bloodvessels,  and  is  known  as  the 
dental  sac. 

The  papilla,  thus  invested  by  the  enamel  organ,  assumes 
the  general  shape  of  the  crown  of  the  future  tooth  :  the 
corpuscles  near  the  surface  become  more  numerous  and 
larger  than  those  of  the  interior,  and  the  layer  of  odonto- 
blasts is  formed,  after  which  the  deposition  of  dentine  is 
begun.  Important  changes  have  in  the  meantime  taken 
place  in  the  enamel  organ  :  the  cells  upon  the  concave  sur- 


168  PART    II.      HISTOLOGICAL  ANATOMY. 

face  next  the  papilla  become  greatly  elongated  verticall}'', 
and  b}^  their  activity  begin  the  deposition  of  the  enamel ; 
whether  b}'-  a  process  of  secretion  or  by  the  transforma- 
tion of  their  own  substance  is  still  a  matter  of  debate ; 
the  layer  of  enamel-forming  cells  is  known  as  the  inner 
layer  of  the  enamel  organ :  the  margin  of  this  layer  is 
continued  over  the  convex  surface  of  the  enamel  organ  b}^ 
the  outer  layer  of  cuboidal  epithelial  cells,  which  is  in  turn 
continuous  through  the  stalk  with  the  epithelium  of  the 
jaw,  and  which  lies  in  contact  with  the  dental  sac:  the 
interior  of  the  enamel  organ  contains  a  mass  of  cells,  the 
so-called  middle  layer,  which  becomes  converted  into 
branching  corpuscles,  their  interspaces  being  filled  with  a 
water}'  fluid.  From  the  side  of  the  enamel  organ  or  from 
the  adjacent  stalk  a  bud  arises  early  in  the  history  of  the 
tooth,  and  grows  downward  to  form  a  second  enamel 
organ  in  the  case  of  those  teeth  which  are  succeeded  by 
others. 

As  the  time  of  irruption  approaches,  the  middle  la^^er  of 
the  enamel  organ  becomes  greatly  compressed,  and  the 
enamel  forming  cells  of  the  inner  layer  reduced  in  depths 
to  flattened  scale-like  bodies,  forming  the  membrane  of 
Nasmyth.  The  papilla  rapidly  elongates,  forming  the 
fang,  thus  pushing  the  first  formed  crown  upward  to- 
ward its  final  position.  After  the  dentinal  portion  of  the 
fang  is  fully  formed  the  cementum  is  deposited  upon  its 
outer  surface  in  a  manner  practically  the  same  as  that  of 
the  formation  of  bone  elsewhere. 

The  tongue    is    a  mass  of  striped  muscular  tissue  in- 


CttAl'TlvR  XIV.      MOITH  AND  CONTEMTS.  100 

Vested  with  the  niueous  njembrane  of  tlic  mouth,  which 
upon  its  upper  surface  is  specially  modified.  The  muscu- 
lar fibres  which  make  up  its  bulk  are  divided  into  two 
symmetrical  masses  by  a  median  vertical  partition  of 
fibrous  tissue,  the  lingual  septum,  which  is  better  devel- 
oped below  than  above:  right  and  left  of  this  the}' lie  in 
interwoven  bundles  some  of  which  run  kHigitudinall}^ 
others  transversely,  and  a  third  set  vertically;  their 
arrangement  to  form  the  lingual  muscles  is  a  matter 
anatomical  rather  than  of  histological  discussion. 

The  mucous  membrane  of  the  lower  surface  and  sides  of 
the  tongue  does  not  differ  essentiall)'  from  that  of  the  rest 
of  the  mouth.  At  the  margin  of  the  upper  or  dorsal  sur- 
face, however,  it  suddenly  becomes  modified,  the  majority 
of  the  papillae  becoming  greatly  enlarged  to  form  the  pro- 
jections of  the  surface  visible  to  the  naked  ey^  and  known 
as  the  lingual  papillae.  Of  these  there  are  three  kinds : 
those  most  abundant,  and  covering  the  whole  surface  of 
the  tongue  with  a  velvety  layer,  are  known  as  conical  or 
filiform  from  their  shape;  they  are  tapering  upgrowths 
of  the  corium,  covered  with  a  corresponding  layer  of  epi- 
thelium, and  are  frequently  bifid  at  their  tips:  scattered 
here  and  there  among  them  are  spheroidal  elevations  of 
the  surface  about  the  size  of  a  small  pin's  head  ;  these  are 
termed  fungiform^  and  are  invested  with  a  thin  layer  of 
stratified  epithelium.  Near  the  root  of  the  tongue,  and 
arranged  in  a  V-shaped  figure  whose  apex  is  directed  back- 
ward, are  eight  to  twelve  large  papillae,  roughly  cylindrical 
in  form,  which  are  sunk  in  depressions  of  the  surface  and 
surrounded  by  circular  trenches  or  grooves  ;  these  are  the 


170  PART    II.      HISTOLOGICAL  ANATOMY, 

cir  cum  vallate  papillae:  the  stratified  epithelium  of  their 
sides  forms  a  deep  la^'er  in  which  are  imbedded  the  taste- 
buds  or  gustatory  organs.  The  description  of  these  struc- 
tures will  be  given  in  the  chapter  devoted  to  the  organs  of 
special  sense. 

The  papillae  all  contain  capillary  networks,  the  fungi- 
form and  circumvallate  papillae  having  a  specially  abun- 
dant blood  supply.  The  mucous  membrane  of  the  pos- 
terior part  of  the  tongue  contains  diffuse  adenoid  tissue 
with  here  and  there  occasional  nodules  of  the  same. 
The  lingual  glands  of  the  anterior  part  of  the  tongue  are 
all  of  the  mjLicous  type :  some  of  those  of  the  posterior 
part,  known  as  the  glands  of  Ebner,  are  of  the  serous 
type,  their  ducts  opening  into  the  grooves  which  surround 
the  circumvallate  papillae.  The  tongue  is  formed  in  part  by 
an  upgrowth  from  the  floor  of  the  mouth,  in  part  by  a 
growth  forward  from  the  ventral  wall  of  the  pharynx : 
its  epithelium  is  therefore  in  part  epiblastic  and  in  part 
hypoblastic  in  origin ;  but  the  two  regions  cannot  be  dis- 
tinguished in  the  adult 


iJo^^^Uii^  M^^^       vhiwvt 


CHAPTER  XV.      ALIMENTARY    CANAL.  171 


CHAPTER  XV. 
THE  ALIMENTARY  CANAL. 


As  we  pass  from  the  mouth  through  the  opening  of  the 
fauces  we  enter  the  tube  lined  with  h  vpoblastic  epitheHum 
of  which  the  mouth  is  the  antechamber,  and  to  which  the 
name  of  the  alimentary  canal  is  strictly  applicable.  The 
anterior  and  posterior  pillars  of  the  fauces  are  folds  of  the 
mucous  membrane  rich  in  mucous  glands  and  containing 
some  diffuse  adenoid  tissue :  between  them  on  either  side 
of  the  mouth  is  situated  a  tonsil:  a  rounded  body  of  va- 
riable size  consisting  of  a  mass  of  fibrous  tissue  richly  in- 
filtrated with  adenoid  tissue  and  containing  a  variable 
number  (from  ten  to  twenty)  well-defined  nodules  of  the 
same  structure,  like  those  found  in  the  mucous  membrane 
of  the  rest  of  the  tongue.  The  surface  of  the  adenoid  mass 
-thus  formed  is  covered  with  stratified  epithelium  continu- 
ous with  that  adjacent,  which  is  in  places  invaded  by 
numbers  of  leucocytes  passing  through  it  into  the  oral 
cavity  to  become  the  bodies  described  above  as  salivar\' 
corpuscles.  Deep  folds  or  depressions  of  the  surface  occur, 
lined  with  stratified  epithelium :  these  are  termed  crypts, 
and  receive  the  secretions  of  subjacent  mucous  glands. 

The  alimentary  canal,  p'taperWW^c^lVeiil  is  a  tube  which, 
beginning  at  the  pharynx  and  ending  with  the  rectum. 


172  PART    II.      HISTOLOGICAL   ANATOMY. 

varies  in  size  and  form  in  different  portions.  Its  structure 
varies  correspondingly  in  detail,  as  will  be  stated  in  the 
descriptions  of  the  various  regions.  In  general  plan  it  is 
a  tube  having  essentially  a  double  wall  whose  divisions 
are  connected  by  a  stratum  of  areolar  tissue,  and  invested 
throughout  a  portion  of  its  extent  b}' the  serous  membrane 
which  lines  the  abdominal  cavity. 

The  inner  or  glandular  layer,  the  mucosa,  is  a  mucous 
membrane  whose  epithelium  varies  greatly  in  different 
parts :  the  fibrous  portion  is  in  all  the  various  regions  more 
or  less  infiltrated  with  adenoid  tissue  which  is  in  places 
quite  scanty  in  amount  or  wanting  altogether,  in  places 
so  abundant  as  to  form  conspicuous  nodular  masses. 
Smooth  muscular  fibres  are  also  present  in  all  the  regions, 
usualh'-  in  such  quantity  as  to  form  a  well-defined  muscu- 
laris  mucosae,  which  may  consist  of  two  or  even  three 
distinct  layers. 

The  stratum  of  areolar  tissue  into  which  the  loose  and 
folded  mucosa  passes,  the  submucosa,  varies  in  extent  in 
different  regions,  but  always  permits  of  free  movement  of 
the  folds  of  the  mucosa.  It  is  characterized  throughout 
the  greater  portion  of  its  extent  b}'  the  presence  of  a  net- 
w^ork  of  nonmedullated  nervous  fibres  with  small  ganglia 
at  the  nodes,  the  plexus  of  Meissner:  and  contains  the 
larger  blood  and  lymph  vessels  from  w^hich  those  of  the 
mucosa  are  derived. 

Beyond  the  submucosa,  which  is  continuous  with  its  in- 
ternal skeletal  framework,  is  the  outer  principal  layer  of 
the  tube,  the  musculosa,  or  muscular  layer.  The  anterior 
portion  is  composed  of  striped  fibres;   the  remainder  (and 


chaptp:r  XV.    ali.mkntary  canal.  173 

greater  portion)  of  sniooth  filires.  Except  in  the  pharynx, 
where  definite  muscles  are  found,  the  bundles  of  fibres, 
whether  smooth  or  striped,  are  arranp^cd  in  two  continu- 
ous strata,  the  inner  of  which  is  composed  of  circular  and 
the  outer  of  longitudinal  fibres:  between  them  is  a  thin 
layer  of  connective  tissue  containing  blood  vessels  and  a 
second  and  larger  nervous  network  known  as  the  plexus 
of  Auerbach. 

In  the  neck  atid  thorax  the  connective  tissue  adjacent 
forms  a  more  or  less  well  defined  membranous  layer  just 
without  the  musculosa  sometimes  described  as  a  separate 
layer  of  the  canal  under  the  name  of  the  fibrosa;  while  the 
term  serosa  is  commonly  applied  to  the  peritoneal  fold 
which  rests  upon  the  musculosa  throughout  the  abdomi- 
nal cavity. 

The  pharnyx  is  the  first  region  of  the  alimentary  canal. 
Its  u_pper  or  res^giratory  portion  is  lined  with  ciliated  col- 
umnar epithelium  ;  while  that  of  the  lower  and  larger  por- 
tion is  stratified  squamous:  the  latter  is  throughout  a  large 
part  of  its  extent  more  or  less  infiltrated  with  leucocytes. 
The  fibrous  membrane  contains  a  large  amount  of  adenoid 
tissue,  which  in  the  upper  and  posterior  part  of  the  phar- 
ynx is  collected  into  a  large  patch  containing  adenoid  nod- 
ules and  crypt-like  depressions,  to  which  the  name  of  the 
pharyngeal  tonsil  has  been  given  :  gjan^s  of  the  rnucous 
type  are  abundant.  Beneath  the  submucosa  is  found  a 
dense  layer  of  fibrous  tissue,  the  pharyngeal  aponeurosis, 
separating  the  mucous  membrane  from  the  muscular  coat. 
Without  are  the  well  defined  pharyngeal  muscles,  com- 


u   0  IkAj^  ^JLCuf4- H  odj^\AMM}LL{--uJ (lMy\^i.nA 


174  PART    II.      HISTOLOGICAL  ANATOMY. 

posed  of  striped  muscular  fibres.  The  structure  of  the 
pharynx  ma}^  perhaps  be  best  understood  if  we  regard  the 
musculosa  proper  of  the  aHmentary  tract  as  entirely  defi- 
cient, the  aponeurosis  representing  the  fibrosa,  and  the 
constrictor  muscles  as  external  additions  thereto. 

The  oesophagus,  like  the  lower  part  of  the  pharynx,  is 
lined  with  stratified  squamous  epithelium.  The  mucous 
membrane,  which  is  thrown  into  longitudinal  folds,  bears 
numerous  small  papillae  upon  the  surface,  covered  by  a 
homogeneous  basement  membrane.  Adenoid  tissue  is  very 
sparingly  present  in  the  mucosa,  but  occasional  solitary 
nodules  may  be  found.  The  muscularis  mucosae  is  want- 
ing in  the  uppermost  part  of  the  oesophagus :  lower  it  at 
-first  appears  in  the  form  of  scattered  longitudinal  bun- 
dles, which  near  the  lower  end  of  the  oesophagus  become 
so  numerous  as  to  form  a  continuous  layer  of  longitudinal 
fibres. 

The  submucosa,  which  is  well  developed,  contains  the 
intrinsic  glands  of  the  oesophagus,  which  are  of  the 
mucous  type;  their  ducts  traverse  the  mucosa :  blood  ves- 
sels and  lymphatics  are  numerous :  the  plexus  of  Meissner 
is  wanting  or  very  scantily  developed  throughout  the 
greater  part  of  the  oesophagus. 

The  musculosa  consists  throughout  the  uj22er_third  of  a 
transverse  and  a  longitudinal  layer  of  striped  muscular 
fibres  ;  lower  down  the  layers  are  made  up  wholly  of 
smooth  fibres.  The  plexus  of  Auerbach  is  present,  though 
its  ganglia  are  smaller  and  less  numerous  than  those  of 
the  rest  of  the  alimentarv  canal.    There  is  a  more^iDi^  less 


CHAPTER    XV.      ALIMENTARY   CANAL.  175 

well  defined   fibrosa  just  without   the  external  muscular 
layer. 

The  oesophagus  opens  into  the  stomach,  a  saccular  dila- 
tion of  the  gilimentary  canal  whose  wall  is  greatly  modi- 
fied, chiefly  in  the  mucosa:  thjs  layer  is  much  thjckened, 
owing  to  the  development  of  long  tubular  glands  which 
open  upon  its  surface,  are  imbedded  in  it,  and  make  up  the 
greater  part  of  its  substance ;  it  is  thrown  into  numerous 
folds,  or  rugae,  and  is  covered  with  a  simple  la\'er  of 
columnar  epithelium  containing  nujnerous  goblet  cells,  the 
transition  between  which  and  the  stratified  epithelium  of 
the  oesophagus  is  abrupt :  its  surface  is  dotted  all  over  with 
the  minute  openings  of  the  gastric  glands  previously 
mentioned. 

The  gastric  glands  are  distinguished  by  their  structure 
into  two  kinds,  designated  according  to  their  position  as 
cardiac  (also  called  peptic),  occupying  the  region  extend- 
ing from  the  oesophageal  opening  to  beyond  the  middle  of 
the  stomach,  and  the  pyloric,  which  occup\^  the  Ipw'er 
third.  The  former  have  short  ducts  lined  with  columncir 
epithelium  similar  to  that  of  the  surface  of  the  stomach  : 
the  glandular  portion  is  a  long  wavy  or  slightly  coiled 
tubule,  whose  end  is  sometimes  strongly  bent ;  from  two 
to  four  such  secreting  tubules  commonl}^  opening  into  a 
single  duct :  lining  the  tubule  throughout  its  whole  extent 
is  a  single  layer  of  polyhedral  cells,  while  scattered  along 
the  tubule  here  and  there  between  the  la\'er  of  cells  just 
mentioned  and  the  basement  membrane  are  larger  spher- 
oidal or  ovoidal  cells.    The  cells  first  mentioned  are  called 


UvVMi:*^VAJ^^vyUL\,   «,4^M0U-tUA^   J^  Jm^^UOMU^-^J,^^^^    ithM4^i 


176  PART  II.      HISTOLOGICAL  ANATOMY. 

the  chief  or  from  their  position  the  central  cells,  or,  from 
their  probable  ferment-secreting  function,  the  peptic  cells: 
the  others  are  known  as  the  accessory  or  parietal  cells ; 
they  have  also  been  called  the  oxyntic  cells,  and  are  sup- 
posed to  produce  the  acid  of  the  gastric  juice. 

The  pyloric  glands  differ  from  the  cardiac  in  the  propor- 
tions of  their  parts,  the  ducts  being  much  longer  and  the 
tubules  shorter:  in  the  form  of  the  secreting  tubules, 
which  are  much  more  contorted;  and  in  the  absence  of 
parietal  cells.  Between  the  region  of  well  defined  cardiac 
and  pyloric  glands  is  a  narrow  intermediate  zone  of 
transition  in  which  the  ducts  become  longer  and  the 
tubules  shorter  than  in  the  cardiac  region,  while  the  pari- 
etal cells  become  less  numerous  and  finally  are  wanting 
altogether. 

The  interstices  between  the  gastric  glands  are  filled  with 
interstitial  areolar  tissue  containing  a  rich  network  of 
capillaries  and  lymphatics  and  a  small  amount  of  adenoid 
tissue,  with  occasional  slender  bundles  of  smooth  muscu- 
lar fibres.  About  the  bases  of  the  secreting  tubules  the 
adenoid  tissue  is  more  abundant,  and  here  and  there  forms 
small  nodules  with  ill-defined  boundaries.  Below,  there  is 
a  well-developed  muscularis  mucosae,  consisting  of  two 
distinct  layers,  an  inner  transverse  and  an  outer  longitu- 
dinal layer :  the  inner  is  to  some  extent  subdivided  into 
two  laminae,  the  fibres  of  one  running  somewhat  ob- 
liquely to  those  of  the  other;  from  its  inmost  surface  the 
slender  bundles  above  mentioned  as  running  up  between 
the  gastric  glands  are  given  off. 


CHAPTKR    XV.      ALIMENTARY   CANAL.  177 

The  submucosa  of  the  stomach  is  a  rather  thick  layer  of 
areolar  tissue  which  extends  upwards  into  the  folds  of  the 
mucosa  which  form  the  rugae.  It  contains  the  larger 
bloodvessels  and  lymphatics  connected  with  the  vascular 
supply  of  the  niucosa,  and  a  well  developed  plexus  of  Meiss- 
ner  situated  nearer  to  the  mucosa  than  to  the  muscular 
coat. 

The  musculosa  is  thicker  in  the  stomach  thqji  in  any 
other  portion  of  the  alimentary  canal,  the  increase  in 
thickness  being  chiefly  in  the  inner  or  transverse  coat. 
This,  in  addition  to  being  greatly  increased  in  volume  is  in 
the  cardiac  portion  more  or  less  clearly  divided  into  two 
layers  lying  obliquely  to  each  other  and  to  the  longitudi- 
nal coat.  The  plexus  of  Auerbach  is  conspicuously  seen 
between  the  two  coats.  On  the  outer  surface  is  the  serosa, 
a  thin  fibrous  membrane  covered  by  endothelium,  and  ad- 
hering closely  to  the  longitudinal  muscular  coat  except 
along  a  narrow  strip  at  the  attachment  of  the  mesogaster 
and  the  omentum. 

As  we  pass  from  the  stomach  to  the  duodenum  the  mu- 
cosa becomes  much  thinner,  the  glands  present  in  the 
upper  portion  of  the  latter  bemg  situated  in  the  submu- 
cosa; a  narrow  zone  of  transition  is  seen  between  the 
pyloric  glands  of  the  stomach  and  the  duodenal  glands,  or 
glands  of  Brunner.  The  mucosa,  which  is  thrown  into 
folds,  the  valvulae  conniventes,  is  covered  with  finger-like 
or  leaf-like  projections,  the  villi,  between  which  are  tubu- 
lar depressions,  the  intestinal  follicles  or  crypts  of  Lie- 
berkuhn :  the  siirface  of  the  villi  and  the  lining  of  the  crypts 
are  alike  covered  with   simple  columnar  epithelium  con- 


178  part;  II.      HISTOLOGICAL  ANATOMV: 

taining  large  numbers  of  goblet  cells,  the  columnar  cells 
having  vertically  striated  cajps  or  borders.  Beneath  the 
epithelium  of  both  crypts  and  villi  is  an  endothelial  base- 
ment membrane,  sometimes  called  the  subepithelial  endo- 
thelium of  De  Bove. 

The  centre  of  a  villus  contains  a  lymphatic  with  a 
widely  opened  extremity :  around  it  are  scattered  bundles 
of  smooth  muscular  fibres  proceeding  from  the  muscularis 
mucosae  below :  near  the  surface  is  a  network  of  capil- 
laries formed  from  the  breaking  up  of  a  small  artery  at  the 
base  of  the  villus  and  uniting  to  form  a  vein  near  its  tip : 
the  interstices  are  filled  with  loose  adenoid  tissue.  Between 
the  crypts  at  the  base  of  the  villi  is  a  rich  network  of  capil- 
laries and  Ij'mphatics  and  imbedded,  like  those  of  the  villi, 
in  adenoid  tissue,  which  becomes  quite  dense  about  the 
bases  of  the  crypts ;  here  and  there  well  defined  nodules 
are  found. 

Immediatel}'^  beneath  the  layer  of  adenoid  tissue  upon 
which  the  crypts  abut  is  situated  the  muscularis  mucosae, 
which  is  not  so  thick  as  in  the  stomach,  but  presents  two 
distinct  layers,  an  inner  transverse  and  an  outer  longitu- 
dinal. 

The  submucosa  of  the  upper  portion  of  the  duodenum  is 
quite  thick:  in  addition  to  the  blood  vessels  and  lymphatics 
and  the  plexus  of  Meissner,  it  contains  the  glands  of 
Brunner  already  mentioned.  These  are  compound  tubu- 
lar glands,  whose  short  alveoli,  lined  with  polyhedral  cells 
surrounding  a  distinct  lumen,  open  into  slender  branching 
ducts  which  pass  to  the  centre  of  the  gland,  there  to  open 


CHAPTER   XV.      ALIMENTARY  CANAI,.  179 

into  the  central  duct :  this,  which  is  lined  with  short  col- 
umnar cells,  penetrates  the  mucosa  to  open  between  the 
villi.  The  glandular  epithelium  resembles  that  of  the 
pyloric  glands,  between  which  and  the  glands  in  question 
there  is  a  distinct  transition  at  the  pylorus :  the  epithelial 
cells  contain  fine  granules  similar  to  those  found  in  the 
cells  of  glands  of  the  serous  type,  which  these  glands  re- 
semble save  in  the  non-essential  feature  of  the  larger  size 
of  the  lumen. 

The  transverse  layer  of  the  musculosa,  as  it  reaches  the 
lower  end  of  the  stomach,  becomes  suddenl}^  thickened  to 
form  a  muscular  ring,  the  pylorus  or  gastro-duodenal 
valve:  passing  into  the  intestine  it  becomes  as  quickly 
reduced  in  thickness,  forming  a  la\^er  much  thinner  than 
that  found  in  the  gastric  wall :  a  similar  diminution  is 
found  in  the  thickness  of  the  longitudinal  layer.  The  mus- 
culosa of  the  small  intestine  does  not  otherwise  differ 
from  that  of  the  stomach,  like  which  it  is  invested  by  a 
serosa  formed  from  the  peritoneum. 

In  the  lower  portion  of  the  duodenum  the  glands  of 
Brunner  are  wanting,  and  thesubmucosa  becomes  greatly 
reduced,  the  canal  assuming  the  structural  character  com- 
mon to  the  greater  portion  of  the  small  intestine,  whose 
division  into  the  regions  recognized  by  the  anatomist  is 
not  marked  by  any  histological  characters.  The  only 
noteworthy  feature  in  this  respect  is  the  specialization 
seen  in  the  quantity  and  disposition  of  the  adenoid  tissue 
of  the  mucosa,  which  attains  its  highest  development  in 
the  lower  portion  of  the  small  intestine,  the  ileum. 


180  PART    II.      HISTOLOGICAL   ANATOMY. 

Throughout  the  whole  of  the  small  intestine  adenoid  tis. 
sue  is  found  in  the  villi  and  between  the  crypts,  as  in  the 
upper  portions  ot  the  duodenum  just  described  :  and  as  in 
that  structure,  there  is  a  continuous  layer  thereof  between 
thecrypts  and  the  muscularis  mucosae.  This,  however,  be- 
comes much  thinner  and  less  dense,  while  here  and  there 
appear  well  defined  nodules  commonly  known  as  solitary 
follicles  or  (by  the  use  of  a  term  equally  unfortunate)  sol- 
itary glands:  it  should  of  course  be  understood  that  these 
bodies  have  functionally  nothing  in  common  with  either 
glands  or  follicles  as  defined  on  a  preceding  page:  they  are 
broadly  pear-shaped  bodies  whose  bases  usually  extend 
into  the  submucosa,  which  is  locally  thickened  in  connec- 
tion therewith,  and  whose  apices  when  large  project  as 
rounded  eminences  upon  the  surface  of  the  mucosa  covered 
with  a  single  layer  of  columnar  epithelium,  both  crypts 
and  villi  being  wanting  at  that  point :  in  structure  each  is 
a  dense  mass  of  adenoid  tissue  in  the  main  uniformly  dis- 
posed, but  with  a  slight  diminution  in  density  toward  the 
centre  of  the  mass,  containing  a  capillary  network  and  sur- 
rounded by  numerous  lymphatics:  the  nodules  are  sur- 
rounded by  the  fibrous  tissue  of  the  mucosa,  which  is  here 
quite  dense,  but  they  have  no  definite  capsule. 

The  solitary  nodules  of  adenoid  tissue  give  place  to  a 
great  extent  in  the  lower  portion  of  the  intestine  to  clus- 
ters of  similar  bodies  sometimes  called  agminated  glands, 
but  better  known  as  Peyer's  patches.  Where  these  occur 
the  mucosa  and  submucosa  are  both  thicker  than  in  other 
portions  of  the  intestine :  the  nodules  are  closely  crowded 
together  and  in  the  centre  of  the  mass  become  confluent : 


CHAPTER    XV.      ALIMENTARY   CANAL.  181 

the  conical  apices  of  most  of  them  project  upon  the  sur- 
face of  the  mucosa:  the  epithelium  of  the  projections  (as 
is  the  case  to  some  extent  with  that  of  the  solitary  nodules 
just  described)  becomes  infiltrated  with  leucocytes  as  in 
the  case  of  that  investing  the  tonsils,  often  to  such  an  ex- 
tent as  to  be  no  longer  clearly  distinguishable,  large  num- 
bers of  leucocytes  thus  constantly  entering  the  intestinal 
cavity. 

At  the  margin  of  the  folds  which  constitute  the  iko- 
colic  valve  the  villi  cease  abruptly;  and  as  we  pass  the 
margin  in  question  we  come  upon  the  wall  of  the  large 
intestine,  including  its  greater  portion,  the  colon,  with 
its  anterior  prolongation,  the  caecum  (ending  in  the  ver- 
miform appendix),  and  its  posterior  continuation,  the 
rectum. 

The  mucosa  of  the  colon,  like  that  of  the  rest  of  the 
large  intestine,  is  devoid  of  villi.  It  is  crowded  through- 
out its  whole  extent  with  tubular  follicles  resembling  the 
crj'pts  of  the  small  intestine,  but  larger  and  showing  ordi- 
narily a  larger  proportion  of  goblet  cells.  The  interstitial 
adenoid  tissue  is  scant)',  but  large  solitary  nodules  occa- 
sionallv  occur.  The  muscularis  mucosae  does  not  differ 
materially  from  that  of  the  small  intestine. 

The  submucosa  is  a  layer  of  moderate  thickness,  con- 
taining the  larger  vessels  and  the  plexus  of  Meissner.  The 
musculosa  is  well  developed  :  the  inner  or  circular  layer  is 
thickened  uniformly,  while  the  outer  layer  is  chiefly  gath- 
ered into  three  longitudinal  bands.  The  outer  surface  is 
covered  for  a  portion  of  its  extent  with  a  serosa. 


182  PART  II.      HISTOLOGICAL   AMATOMY. 

The  caecum  is  essentially  similar  in  structure  to  the 
colon.  The  appendix  vermiformis  is  a  rudimentary  struc- 
ture, variable  in  size  and  development:  its  mucosa  con- 
tains numerous  follicles,  like  those  of  the  colon,  and  also 
solitary  nodules  of  adenoid  tissue:  its  musculosa  con- 
sists of  two  layers,  the  outer  or  longitudinal  differing 
from  that  of  the  colon  in  being  of  uniform  thickness  and 
thicker  than  the  inner  or  circular  laj^er. 

As  the  alimentary^ canal  is  continued  downward  to  form 
the  rectum,  its  structure  undergoes  important  changes  in 
each  of  the  fundamental  layers,  in  connection  with  the 
modification  which  takes  place  in  the  function  of  the  ca- 
nal: these  will  as  before,  be  described  in  connection  with 
the  layers  successively. 

The  mucosa  of  the  rectum,  like  that  of  the  colon,  is^e- 
void  of  villi :  it  is  much  thicker  than  that  of  the  colon,  in- 
creasing in  thickness  downwards  to  the  anal  opening.  In 
addition  to  temporary  foldings,  chiefly  longitudinal,  which 
exist  when  the  tube  is  empty,  there  are  present  three  or 
four  transverse  folds,  semilunar  in  shape,  known  as  the 
valves  of  Houston,  and  near  the  terminus  several  longitu- 
dinal folds,  the  columns  of  Morgagni ;  their  relations  to 
subjacent  structures  will  be  mentioned  later.  The  epithe- 
lium  of  the  greater  portions  of  the  rectum  is  columnar, 
like  that  of  the  small  and  the  large  intestines :  just  above 
the  anus  there  is  in  man  a  transition  from  simple  columnar 
to  stratified  squamous  epithelium  continuous  with  that  of 
the  surface  of  the  body :  this  transition  takes  place  upon 


CHAPTER   XV.      AI.IMENTARY   CANAL.  183 

the  columns  of  Morgagni  higher  up  than  within  the  de- 
pressions which  lie  between  them.  Follicles  or  crypts  like 
those  of  the  colon  abound  in  the  mucosa  of  the  greater 
portions  of  the  rectum  :  the\'  disappear  at  the  level  of  the 
commencement  of  the  columns  of  Morgagni.  In  addition 
to  a  small  amount  of  diffuse  adenoid  tissue,  occasional 
solitary  nodules  arc  found :  they  are  less  numerous  rela- 
tively than  in  the  colon. 

The  fibrous  structure  of  the  mucosa  of  the  rectum  is 
denser  than  that  of  the  colon,  particularly  toward  the 
lower  extremity,  where  there  is  a  marked  increase  in  the 
proportion  of  elastic  fibres  and  a  diminution  in  the  amount 
of  adenoid  tissue,  the  fibrous  layer  finally  passing  without 
abrupt  transition  into  the  corium  of  the  skin.  The  mus- 
cularis  mucosae  is  gradually  increased  in  extent,  and 
consists  chiefly  of  longitudinal  fibres:  at  the  lower  por- 
tion these  are  gathered  together  in  several  stout  bundles, 
which,  with  the  overlying  folds  of  the  mucosa,  form  the 
columns  of  Morgagni  above  described :  these  are  contin- 
ued into  tendinous  bundles  which  are  inserted  into  the 
skin  in  the  immediate  vicinity  of  the  anal  opening. 

The  submucosa  of  the  rectum  becomes  considerably 
thickened  toward  its  lower  extremity',  in  connection  with 
the  increased  mobility  of  the  mucosa,  and  is  in  addition 
reinforced  b\'  the  presence  of  numerous  elastic  fibres :  it 
contains,  moreover,  a  certain  amount  of  adenoid  tissue, 
in  this  respect  differing  markedly  from  the  same  la^-er  in 
other  portions  of  the  canal.  A  distinct  plexus  of  Meissner 
is  present. 


184  PART  11.      HISTOLOGICAL  ANATOMY, 

The  musculosa  of  the  rectum  undergoes  marked  in- 
crease in  thickness  from  above  downward  in  both  the  cir- 
cular and  the  longitudinal  layer.  In  the  former,  there  is  in 
addition  to  the  gradual  increase  in  volume  a  tendency  to 
divide  into  bundles  of  fibres  of  gradually  increasing  size : 
at  its  lowermost  limit  a  number  of  these  bundles  are  ag- 
gregated together  to  form  the  internal  sphincter  muscle 
which  surrounds  the  anal  opening :  distinct  thickenings  of 
the  circular  layer  also  underlie  the  valves  of  Houston.  The 
longitudinal  layer,  as  it  passes  downward,  also  undergoes 
subdivision,  the  resultant  bundles  of  fibres  diverging  more 
and  more  from  the  circular  layer,  and  being  roughly  divisi- 
ble into  three  strata ;  an  inner,  the  bundles  of  which  are 
interwoven  with  those  that  form  the  internal  sphincter ;  a 
middle,  whose  bundles  terminate  along  the  boundary  of 
the  external  sphincter  (an  extrinsic  muscular  structure 
composed  of  striped  fibres) ;  and  an  outer  stratum  whose 
bundles  unite  with  the  levatores  ani  and  the  recto-coccy- 
geal  muscles. 


The  extrinsic  glands  connected  with  the  alimentary  canal 
are  but  two  in  number,  the  pancreas  and  the  liver,  both  of 
them  opening  into  the  duodenal  portion  of  the  small  intes- 
tine, of  which  they  are  outgrowths.  The  pancreas  is  in 
most  respects  similar  in  structure  to  a  large  salivary  gland 
of  the  serous  type;  its  connective  tissue  framework  as  well 
as  its  blood  and  lymph  vascular  supply  being  the  same, 
and  the  form  of  the  alveoli  similar,  as  well  as  their  arrange- 


4, 


CHAPTER    XV.      ALIMKNTARY    CANAL.  185 

ment  in  lobules  and  lobes.  The  epithelium  of  the  alveoli  re- 
sembles that  of  a  serous  {.^land  in  its  j^ranular  appearance, 
and  in  the  position  of  the  nucleus;  the  lumen  is  also  very 
small.  Certain  differences  are,  however,  to  be  noted.  The 
ajveoli  are  as  a  rule  longer  and  more  generally  tubular  in 
form  :  theepithelial  cells  are  taller  and  narrow^er,  as  a  rule, 
approaching  to  the  columnar  form  :  there  have  been  de- 
scribed under  the  name  of  centroacinar  cells,  certain  spin- 
dle-shaped elements  occupying  the  lumen  near  the  union 
of  the  alveolus  with  the  intermediate  tubule;  their  origin 
and  function  are  alike  uncertain.  The  intermediate  tubules, 
with  their  characteristic  flattened  epithelium,  are  more 
extensive  than  in  the  salivary  glands,  replacing  in  great 
measure  the  intralobular  ducts.  The  pancreas  is  also 
characterized  by  the  presence  between  the  alveoli  of  pecu- 
liar groups  or  clusters  of  intertubular  cells  which  form 
nodular  masses  situated  in  the  intralobular  connective 
tissue  or  the  interlobular  septa;  each  mass  having  a  defi- 
nite tuft  of  capillaries :  these  bodies  whose  nature  is  not 
clearly  understood,  are  by  some  regarded  as  newly  formed 
or  embryonic  alveoli. 

The  liver  is  at  once  the  largest  and  the  most  highly 
modified  of  all  the  digestive  glands.  While  it  must  be  re- 
garded as  formed  by  the  modification  of  a  primarily  tubu- 
lar structure,  the  multiplication  of  the  glandular  epithe- 
lial cells  has  obliterated  the  lumen  of  the  tubule  in  each 
instance,  the  secretion  formed  by  the  epithelium  being  dis- 
charged throughtheagency  of  intercellular  channels  which 
are  unlike  anything  found  in  any  other  portion  of  the 
human  body.     This  extreme  modification  in  the  arrange- 


186  PART  n.      HISTOLOGICAL  ANATOMY. 

ment  of  the  glandular  epithelium  is  also  accompanied  by 
an  equally  great  modification  in  the  arrangement  of  the 
blood  vessels,  the  capillary  network  apparently  penetrat- 
ing the  epithelial  masses ;  thus  forming  what  at  first  sight 
seems  to  be  an  exception  to  the  general  rule  that  epithelia 
are  devoid  of  blood  vessels.  The  structure  of  the  liver  will 
therefore  best  be  understood  if  we  begin  by  examining  the 
blood  supply  and  the  framework  of  skeletal  tissue  with 
which  it  is  associated. 

The  surface  of  the  liver  is  invested  by  a  thin  capsule  of 
fibrous  tissue,  surmounted  by  the  peritoneal  endothelium. 
From  this  capsule  thin  lamellae  penetrate  the  surface  of 
the  gland  as  interlobular  septa :  in  the  transverse  fissure 
it  becomes  continuous  at  the  porta  (which  corresponds  to 
the  hilum  of  an  ordinary  gland)  with  an  important  in- 
growth of  connective  tissue,  the  so-called  capsule  of  Glis- 
son,  which  forms  by  far  the  larger  part  of  the  interlobular 
tissue,  becoming  continuous  with  the  ingrowths  from  the 
capsule.  The  porta  is  the  place  of  entrance  for  the  nerve- 
supply  of  the  liver,  and  for  the  large  portal  vein  and  the 
smaller  hepatic  artery ;  as  well  as  of  exit  for  the  bile 
duct  and  for  the  principal  lymphatic  trunk :  these  four 
vessels  and  their  ramifications,  together  with  strands  of 
nerve-fibres,  are  usually  found  in  close  proximity,  sur- 
rounded by  a  certain  amount  of  connective  tissue,  the 
whole  constituting  what  is  known  as  a  portal  canal. 
The  blood  leaves  the  liver  by  means  of  the  hepatic  veins, 
whose  branches  are  formed  by  the  union  of  the  sublobular 
veins,  the  origin  of  which  will  be  presently  described  :  the 
sublobular  and  hepatic  veins  are  not  accompanied  in  their 


CHAPTER    XV.      ALIMENTARY   CANAL.  187 

course  by  branches  of  other  vessels,  a  character  by  means 
of  which  they  can  be  readily  distinguished  from  the 
branches  of  the  portal  vein. 

The  lobules  of  the  liver,  sometimes  (but  less  correctly) 
termed  the  acini  or  alveoli,  arc  masses  of  hepatic  cells, 
penetrated  by  capillary  networks,  polyhedral  in  form  as 
the  result  of  pressure,  and  in  man  something  over  a  milli- 
metre in  diameter.  Those  at  the  surface  of  the  liver  have 
the  axis  vertical  or  nearly  so  to  that  surface:  but  there  is  no 
such  regularity  of  position  in  the  case  of  those  more 
deeply  situated.  The  interlobular  septa  are  in  a  few  mam- 
mals (of  which  the  pig  is  one)  quite  stout  and  conspicuous : 
in  man  they  are  far  less  developed  ;  and  the  boundaries  of 
the  lobules  are  in  consequence  not  always  readily  deter- 
mined. From  certain  points  on  the  septa  a  very  scanty 
framework  of  connective  tissue  traverses  the  interior  of 
the  lobule,  accompanying  the  capillaries:  its  presence  even 
in  small  quantity  is  a  fact  of  importance  in  forming  a  clear 
conception  of  the  essential  structure  of  the  liver. 

The  branches  of  the  portal  yein  end  in  veinlets  situated 
in  the  septa  and  hence  known  as  interlobular  veins,  each 
lobule  having  several  surrounding  it :  these  give  off  capil- 
laries (in  a  manner  similar  to  arteries  elsewhere)  which 
form  a  network  whose  meshes  converge  toward  the  axis 
of  the  lobule :  this  is  occupied  by  a  single  vessel,  the  cen- 
tral or  intralobular  vein,  into  which  the  capillaries 
empty.  The  intralobular  vein  passes  at  the  base  of  the 
lobule  into  a  sublobular  vein,  the  hepatic  veins  being  as 
has  been  stated,  formed  by  the  confluence  of  the  sublobu- 
lar veins. 


188  PART    II.      HISTOLOGICAL  ANATOMY. 

The  branches  of  the  hepatic  artery  accompany  those  of 
the  portal  vein  to  the  lobules,  where  they  divide  into  inter- 
lobular arterioles  situated  in  the  septa,  together  with  the 
interlobular  veins:  like  the  latter,  they  terminate  in  capil- 
laries ;  these  are,  however,  situated  chiefly'  in  the  connec- 
tive tissue  of  the  septa  and  the  walls  of  the  veins,  a  por^ 
tion  of  them  only  penetrating  the  lobules  for  a  sufficient 
distance  to  form  channels  of  communication  with  the 
capillary  network  of  the  lobule,  as  a  means  of  discharge 
of  the  blood  from  the  arterioles.  The  portal  vein  and  its 
final  branches,  the  interlobular  veins,  are  therefore  to  be 
resfarded  as  the  channels  bv  means  of  which  the  blood  is 
submitted  to  the  glandular  action  of  the  hepatic  cells ;  the 
hepatic  artery  being  probably  in  large  part,  if  not  alto- 
orether  the  channel  of  nutrition  for  the  skeletal  framework 
and  vascular  mechanism  of  the  organ. 

The  network  of  capillaries  with  its  accompanying  scanty' 
connective  tissue  forming,  according  to  some  observers, 
perivascular  lymph  channels,  is  interwoven  with  another 
and  somewhat  coarser  network,  that  of  the  strands  of 
liver  cells  or  hepatic  cords.  These  are  polj'hedral  epithelial 
cells  about  twentj^  micra  in  average  diameter, with  granu- 
lar protoplasm  and  central  spherical  nuclei,  which  perform 
the  complex  secretor}^  function  of  the  liver.  The  size  of 
these  cords  are  such  that  nearly  every  cell  is  brought  at 
some  point  in  contact  with  a  capillary  ;  the  surface  of  the 
cell  being  in  many  cases  slightly  excavated  along  the  line 
of  contact. 

Between  the  hepatic  cells  is  found  a  network  of  minute 
passages  usually  not  more  than  a  micron  in  diameter,  the 


CHAPTER    XV.      ALIMENTARY    CANAL.  189 

so-called  bile  capillaries  or,  as  they  are  otherwise  termed 
the  bile  canaliculi :  these  are  so  situated  that  they  tra- 
verse the  surface  of  the  cell  along  a  side  or  an  angle  oppo- 
site that  in  contact  with  the  blood  capillaries,  never  pass- 
ing loetw^een  the  latter  and  the  cells.  At  the  surface  of  the 
lobule  the  bile  canaliculi  communicate  with  small  inter- 
lobular bile  ducts  whose  flattened  epithelium  is  continu- 
ous with  the  hepatic  cells,  and  whose  basement  membrane 
is  resolved  into  the  scant}*  connectiv'e  tissue  of  the  lobule. 
These  unite  as  they  pass  to  the  portal  canals  to  form  the 
smaller  bile  ducts  situated  therein  ;  the  epithelium  becom- 
ing columnar  and  the  basement  membrane  stouter.  As 
the  smaller  bile  ducts  come  together  to  form  the  larger 
trunks  found  in  the  more  central  region  of  the  liver  the 
cells  of  the  columnar  epithelium  become  taller  and  are 
seen  to  rest  upon  a  distinct  membrana  propria:  while  the 
fibrous  layer  beneath  is  reinforced  by  smooth  muscular 
fibres:  the  largest  ducts  have  in  addition  srnall  mucous 
glands  in  their  walls.  The  structure  of  the  duct  leading 
to  the  intestine,  and  of  the  gall  bladder  as  well,  is  essen- 
tially the  same  as  that  of  the  largest  ducts  within  the 
liver. 

If  we  recall  the  structure  of  a  gland  of  the  serous  type  it 
will  be  remembered  that  the  epithelial  cells  which  line  the 
alveoli  are  so  large  as  to  reduce  the  lumen  to  a  very  slen- 
der and  often  imperceptible  channel  between  their  apices: 
the  bile  canaliculi  may  with  propriety  be  compared  to 
such  channels.  The  alveoli  of  ordinary  glands  may  be 
either  spheroidal  or  elongated  and  tubular,  the  interstices 
between  them  containing  a  variable  amount  of  connective 


190  PART    II.      HISTOLOGICAL   ANATOMY. 

tissue  which  supports  a  capillary  network,  whose  meshes 
conform  to  the  structure  and  arrangement  of  the  alveoli. 
In  the  lower  vertebrates  the  liver  consists  of  tubular  alve- 
oli essentially  similar  to  those  of  other  glands.  If  we  con- 
ceive of  such  tubules  in  the  mammalian  liver  as  becoming 
elongated  and  branched,  and  finally  anastomosing,  we 
shall  understand  the  structure  and  arrangement  of  the 
hepatic  cords.  According  to  this  view  the  anastomoses 
formed  obliterate  the  boundaries  of  the  alveoli ;  but  we 
may  regard  as  belonging  to  one  such  division  all  the  bile 
canaliculi  with  their  surrounding  epithelium  which  lie 
nearest  to  and  presumably  discharge  into  one  of  the  inter- 
lobular bile  ducts :  the  beginnings  of  the  ducts,  with  their 
flattened  epithelium,  corresponding  in  position  and  struc- 
ture with  intermediate  tubules  or  intercalary  ducts  of 
other  glands :  the  lobule  is  thus  seen  to  be  in  fact  as  well 
as  in  name  a  lobule ;  that  is,  a  collection  of  acini  or  alveoli ; 
and  the  application  of  either  of  the  latter  titles  to  it  is 
misleading.  The  most  peculiar  features  in  the  structure  of 
the  liver  are  those  connected  with  the  blood  supply. 


CrtAPTfiR    XVI.      RESPIRATORY   APPARATUS.  191 


CHAPTER  XVI. 
THE   RESPIRATORY  .APP.VrATUS. 


In  the  description  of  the  alimentary  canal  which  has 
just  been  given,  the  conspicuous  layers,  whose  modifica- 
tions in  the  various  regions  have  been  discussed,  are  those 
most  readily  demonstrable  by  the  knife  of  the  anatomist: 
they  are  those,  moreover,  which  naturally  result  from  the 
double  function  of  the  greater  part  of  the  canal,  the  inner 
layer,  or  mucosa,  being  in  substance  a  glandular  layer  by 
means  of  which  the  nutritious  portion  of  the  food  is 
brought  into  a  condition  suitable  for  the  accompanying 
process  of  absorption;  the  outer  layer,  or  musculosa, 
being  in  effect  a  mechanism  by  which  the  food  mass  is 
caused  to  traverse  this  glandular  surface;  while  the  sub- 
mucosa  makes  possible  the  necessary  movements  of  the 
mucosa  and  the  musculosa  on  each  other.  We  should, 
however,  bear  in  mind  the  continuity  of  the  submucosa 
with  the  fibrous  structure  of  the  mucosa  on  the  one  hand, 
and  on  the  other  with  the  interstitial  connective  tissue  of 
the  musculosa,  and  through  it  with  the  fibrosa  or  serosa: 
considering  this,  and  bearing  in  mind  the  embryonic  origin 
of  the  tissues  present,  it  will  be  quite  clear  that  another 
very  natural  division  of  the  wall  of  the  alimentary  canal, 
based  on  histological  rather  than  anatomical  characters, 
would   recognize   two   primary  la\"ers  of  tissues  (differing 


192  PART    II.      HISTOLOGICAL   AN'ATOMY. 

vastly  in  extent),  the  epithelial  layer,  hypoblastic  in  its 
origin;  and  the  musculo-skeletal  (and  vascular)  layer  de- 
rived from  the  mesoblast,  extending  from  the  basement 
membrane,  just  beneath  the  epithelium  w^ithin,  to  the 
fibrosa  (or,  in  the  intestine,  to  the  fibrous  basis  of  the 
serosa)  without:  the  presence  of  this  fibrous  layer  as  one 
of  the  constituents  of  the  alimentary  canal  should  be 
clearh'  borne  in  mind. 

It  is  by  an  outgrov^'th  from  the  phar3'ngeal  region  of 
the  alimentary  canal  that  the  respiratory  apparatus  is 
formed,  and  its  structure  throughout  its  whole  extent  is  de- 
rived from  such  modifications  of  a  musculo-skeletal  layer 
lined  with  epithelium  as  are  required  for  the  proper  per- 
formance of  the  functions  of  the  various  regions.  These 
are  in  the  trachea  and  bronchial  tubes  such  as  will  serve  to 
maintain  the  patency  of  these  channels  for  the  passage  of 
the  air  and  in  some  measure  to  regulate  the  same:  and  in 
the  airsacs  such  as  will  permit  the  freest  interchange  of 
gases  between  the  air  which  they  contain  and  the  capil- 
laries within  their  walls. 

The  trachea  is  plainly  seen  to  be  composed  of  three 
well-defined  layers,  similarly  disposed  to  those  which  ap- 
pear as  the  chief  factors  of  the  alimentary  canal.  The 
inner  of  these  is  the  mucosa;  it  differs  in  some  important 
respects  from  the  layer  of  that  name  in  the  region  just 
mentioned.  It  is  lined  with  w^hat  is  usually  termed  a 
stratified  columnar  ciliated  epithelium:  the  layer  is  but 
a  few  cells  deep ;  those  at  the  surface  are  cylindrical  or 
prismatic,  with  tapering  bases,  their  free  extremities  in 
most  cases  beset  with  numerous  fine  cilia,  whose  united 


ClIAPTHR     XVI.       RHSI'IKATOKV    APPARATUS.  193 

action  causes  constant  currents  toward  the  mouth;  here 
and  there  g^ohlet  cells  occur;  beneath  the  superficial  cells, 
or  intcrsjjersed  between  their  bcises,  are  elongated  and 
more  or  less  spindle-shaped  cells,  which  are  probably  des- 
tined to  replace  them  ;  still  lower  are  pyriform  and  spher- 
oidal cells,  many  of  which  multiply  rapidly  by  cell-divi- 
sion, replacing  the  older  superficial  cells  as  occasion  re- 
quires, resembling  thus  in  function  the  deepest  portion  of 
the  stratified  stiuamous  epithelium  of  the  oesophagus. 

The  epithelium  of  the  trachea  rests  upon  a  basement 
membrane,  a  homogeneous,  elastic  layer  of  considerable 
thickness,  perforated  by  occasional  fine  canals.  This  is 
followed  abruptly  by  a  fibrous  layer,  whose  bundles  are 
irregularly  and  somewhat  loosely  disposed  and  are  inter- 
mintjled  with  a  considerable  number  of  elastic  fibres: 
there  is  a  well  but  not  greatly  developed  network  of  small 
blood  vessels,  accompanied  by  nerves  and  lymphatics,  and 
a  considerable  amount  of  adenoid  infiltration.  Beneath 
the  fibrous  layer,  and  continuous  therewith,  is  the  elastic 
layer,  composed  of  a  dense  network  of  elastic  fibres,  most 
of  which  are  longitudinally  disposed,  which  form  the  inner 
boundary  of  the  mucosa :  it  is  best  developed  in  the  dorsal 
portion  of  the  trachea. 

The  submucosa.  like  that  of  the  alimentary  canal,  is  a 
layer  of  areolar  tissue  serving  to  unite  the  mucosa  with 
the  denser  layer  beyond  it.  In  addition  to  the  larger 
blood  vessels  from  which  the  blood  supply  of  the  mucosa 
is  derived,  and  their  associated  lymphatics,  it  contains 
numerous  small  glands  ol  the  mucous  type,  whose  long 
ducts,  lined  with  cuboidal  epithelium,  traverse  the  mucosa 


194  PART    II.      HISTOLOGICAL  ANATOMY. 

to  open  on  its  surface.  These  glands  are,  as  a  rule,  most 
abundant  in  those  portions  of  the  submucosa  which  are 
situated  opposite  the  intervals  between  the  successive 
fibrous  rings  of  the  trachea.  Here  and  there  in  the  sub- 
mucosa an  occasional  adenoid  nodule  may  be  found,  fre- 
quently associated  with  a  gland  or  its  duct. 

No  single  word  will  readily  indicate  the  complex  struc- 
ture of  the  outer  layer  of  the  trachea :  if  we  for  conven- 
ience make  use  of  the  terra  fibrosa  in  referring  to  it,  we 
shall  designate  it  by  its  most  constant  though  not  most 
conspicuous  factor.  It  is  in  effect  a  tube  of  d^nse  fibrous 
membrane,  rich  in  elastic  fibres,  reinforced  at  regular  in- 
tervals by  the  incomplete  rings  of  cartilage  which  are  the 
most  prominent  components  of  the  trachea,  and  bearing 
on  the  inner  su_rface  of  its  dorsal  portion  the  rudiments 
of  a  muscular  layer.  The  rings  are  composed  of  hyaline 
cartilage  w^hich  is  far  less  brittle  than  usual:  they  are 
imbedded  in  the  fibrous  layer,  which  is  continuous  with 
the  perichondrium  of  each,  and  situated  rather  toward  its 
inner  than  its  outer  limit.  The  muscular  layer  occupies  an 
elongated  rectangular  area  nearly  identical  with  that 
bounded  b}^  the  lines  connecting  the  dorsal  ends  of  the  in- 
complete cartilaginous  rings,  but  somewhat  more  exten- 
sive laterally  :  to  the  band  of  muscular  tissue  thus  formed, 
the  name  of  the  tracheal  muscle  is  given.  It  consists  en- 
tirely of  smooth  muscular  fibres;  the  great  majority  of 
these  are  arranged  in  transverse  bundles  disposed  in  groups 
which  are  separated  by  occasional  transverse  septa  of 
fibrous  tissue ;  external  to  these  are  a  few  thin  and  short 
longitudinal  bundles  forming  an  imperfectly  defined  layer: 


CHAPTER     XVI.      RESPIRATORY   APPARATUS.  195 

the  muscular  laver  is  invested  outwardly  l\v  the  fibrous 
laver  which  it  in  some  measure  replaces,  the  latter  being 
here  much  thinner  than  in  other  portions  of  the  trachea. 
The  bundles  of  the  trans  verse  la  vera  re  inserted  bet  ween  the 
rings  upon  the  fibrous  layer;  those  opposite  the  rings  are 
in  man  inserted  on  their  inner  surface;  in  some  of  the  lower 
mammals  thev  are  inserted  on  the  ends  and  in  others  on 
the  outer  surface  of  the  rings.  The  longitudinal  bundles 
are  inserted  upon  the  ends  of  the  rings  and  upon  the  fib- 
rous layer.  It  is  a  noteworthy  fact  that  in  the  dorsal  por- 
tions of  the  trachea  some  of  the  mucous  glands  are  situ- 
ated external  to  the  muscular  layer,  their  ducts  penetrat- 
ing it  as  well  as  the  other  layers  of  tissue  beneath  the  epi- 
thelium. 

The  bronchi,  or  right  and  left  divisions  of  the  trachea, 
while  they  differ  from  it  anatomically,  particularly  in  the 
form  of  the  cartilages,  resemble  it  in  all  essential  respects 
as  regards  their  histological  structure.  As  the}-  enter  the 
lungs  they  branch  and  subdivide  repeatedly,  their  imme- 
diate continuations  and  their  subdivisions,  with  the  ex- 
ception of  the  smallest,  being  known  as  the  bronchial 
tubes,  or,  as  they  are  sometimes  termed,  the  intrapulmon- 
ary  bronchi:  by  the  time  they  are  reduced  to  a  diameter 
of  one  millimetre,  or  thereabouts,  they  are  known  from 
their  size  and  structure  as  bronchioles;  the  smallest  of 
these  are  never  less  than  half  a  millimetre  in  diameter. 

The  largest  bronchial  tubes  are  essentially  like  the 
extra-pulmonary  bronchi  in  structure :  as  we  pass  toward 
the  bronchioles,  their  structure  undergoes  marked  though 


196  PART  11.      HISTOLOGICAL  ANATOMY. 

gradual  modification:  that  of  a  tube  of  medium  size  may  be 
described  as  follows.  The  epithelium,  like  that  of  the 
trachea  and  larger  bronchi,  is  stratified,  columnar  and 
ciliated,  and  rests  on  a  homogeneous  basement  membrane. 
Beneath  this  is  a  fibrous  layer  containing  a  moderate 
amount  of  adenoid  tissue,  and  numerous  elastic  fibres  no 
longer  forming  a  continuous  layer,  but  gathered  into 
strands,  which  form  the  bases  of  the  folds  into  which  the 
inner  portion  of  the  mucosa  is  frequently  thrown  :  exter- 
nal to  the  adeno-fibrous  layer  is  a  well-defined  muscularis 
mucosae,  composed  of  smooth  fibres  transversely  dis- 
posed, this  layer,  which  is  of  variable  thickness  from 
point  to  point  along  the  tube,  is  sometimes  designated 
the  bronchial  muscle. 

External  to  the  muscular  layer  is  the  submucosa,  com- 
posed here,  as  in  the  trachea  and  bronchi,  of  areolar  tis- 
sue containing  rnucous  glands  and  plexuses.  QJLblood  and 
lymph  vessels.  The  outermost  layer,  that  which  we  have 
for  convenience  termed  the  fibrosa,  is  the  one  which  first 
shows  signs  of  reduction:  the  fibrous  membrane  is  by  no 
means  as  dense,  nor  as  clearly  defined,  either  from  the  sub- 
mucosa within  or  from  the  adjacent  tissues  without,  as  in 
the  trachea;  the  cartilaginous  rings  of  the  latter  are  rep- 
resented by  plates  of  cartilage  of  varying  size  and  shape; 
and  there  is  nothing  present  in  this  laj^er  that  may  be  re- 
garded as  corresponding  structurally  to  the  tracheal  mus- 
cle. Occasional  lobules  of  fat  may  be  seen,  and  the  mucous 
glnnds  not  unfrequently  penetrate  into  this  layer,  which 
thus  tends  to  approach  in  its  structure  that  of  the  sub- 
mucosa. 


CHAPTER    XVI.      UKSIMKATOKY    APPARATUS.  197 

As  we  ])ass  toward  the  smaller  bronchial  tubes  the  epi- 
thelium becomes  gradually  reduced  to  a  single  layer  ot" 
columnar  ciliated  cells  resting  upon  a  basement  membrane. 
The  fibrous  layer  beneath  becomes  much  thin^ner;  the  rela- 
tive amount  of  adenoid  tissue  becomes  less  and  less,  and 
elastic  fibres  become  far  less  numerous,  though  they  do 
not  altogether  disappear.  The  muscular  layer,  on  the 
other  hand,  for  a  time  at  least,  increases  in  relative  thick- 
ness, becoming  one  of  the  niosc  conspicuous  features  of 
small  tubes.  The  submucosa  and  fibrosa  become  blended 
into  one  layer  ^f  loose  fibrous  tissue  rich  in  lymphatics 
and  containing  blood  vessels,  the  mucous  glands  djsap- 
pearing  from  the  former,  together  with  the  cartilaginous 
plcites  (and  the  nodules  which  succeed  them)  from  the  lat- 
ter. 

Within  the  bronchioles  (otherwise  designated  the  term- 
inal bronchi)  still  further  reductions  of  structure  take 
place:  the  epkhelium  changes  from  columnar  to  cubical, 
loses  its  cilia,  and  later  becomes  more  or  less  flattened, 
forming  a  single  layer  of  polyhedral  granular  cells  upon 
the  basement  membrane.  The  latter  rests  on  a  thin  layer 
of  fibrous  tissue  with  longitudinal  elastic  fibres:  the  muscu- 
lar layer  is  reduced  to  scattered  bundles  and  later  to  iso- 
lated fibres,  without  finally  disappearing  altogether;  while 
the  submucosa  and  the  fibrosa  become  blended  with  each 
other  and  with  the  fibrous  layer  of  the  mucosa. 

The  bronchioles  lead  into  larger  pyramidal  or  irregular 
shaped  spaces,  the  infundibula,  into  each  of  which  open 
by  wide  apertures  a  large  number  of  the  spheroidal  or 


198  PART    II.      HISTOLOGICAL   ANATOMY. 

polyhedral  air-sacs  or  alveoli  which  are  the  ultimate 
chambers  of  the  lung:  each  infundibulum  with  its  associ- 
ated alveoli  making  up  one  of  the  component  lobules 
of  that  organ.  The  principal  change  in  passing  from  the 
bronchiole  to  the  infundibulum  is  found  in  the  epithelial 
layer:  the  low  granular  cells  with  which  the  distal  extrem- 
ity of  the  bronchiole  is  lined  are  found  in  the  infundibulum 
in  patches  which  become  smaller  and  less  numerous  as  we 
proceed  to  its  farther  extremity:  between  them  are  found 
larger  and  thinner  transparent  cells  which  form  a  sirnple 
squamous  epithelium  to  which  the  distinctive  title  of  res- 
piratory epithelium  is  applied .  The  basement  membrane, 
the  longitudinal  network  of  elastic  fibres,  and  the  layer 
of  scattered  muscular  fibres  are  continued  without  essen- 
tial change.  The  fibrous  portion  of  the  wall  of  the  bron- 
chiole is  represented  by  scattered  branched  connective  tis- 
sue corpuscles  situated  in  the  interstices  between  the  elas- 
tic and  muscular  fibres. 

The  alveoli  are  in  the  main  continuations  of  the  walls  of 
the  infundibulum.  The  epithelium  consists  almost  entirely 
of  the  large  flat  cells  above  mentioned,  the  smaller  granu- 
lar cells  being  scattered  sparingly  among  them  either  singly 
or  in  groups  of  two  or  three:  between  the  cells,  and  in  par- 
ticular at  the  angle  where  three  or  four  meet,  are  occasional 
stomata,  minute  openings  which  communicate  with  the 
lymph  spaces  below.  About  the  mouth  of  each  alveolus 
there  is  an  annular  bundle  of  elastic  fibres  from  which  is 
given  off  a  network,  which,  together  with  a  small  amount 
of  fibrous  tissue  and  a  few  connective  tissue  corpuscles, 
forms  the  wall  of  the  alveolus  and  the  support  of  the  epi- 


CHAPTER    XVI.      RESPIRATORY   APPARATUS,  199 

theliuni  and  capillary  network:  from  the  form  and  dispo- 
sition of  the  alveoli  it  results  that  a  single  layer  thus 
formed  does  duty  in  jj;^reat  measure  for  two  adjacent  alveoli. 
The  capillary  network  contained  in  the  interalveolar 
septum  thus  formed  isexceedin«i^ly  dense;  and  itsloops  pass 
from  side  to  side  of  the  septum  in  a  serpentine  course,  thus 
l)rin<i^ing  the  blood  contained  within  them  as  near  as  pos- 
sible to  the  air  in  each  ot  the  alveoli. 

The  lymph  spaces  in  the  alveolar  walls  communicate 
with  the  lymphatics  situated  in  the  connective  tissue 
septa  which  lie  between  the  lobules.  Each  lobule,  as  thus 
bounded,  is  irregularly  pyramidal  in  form,  its  apex  being 
situated  at  the  bronchiole,  toward  which  the  limiting 
septa  tend :  peripherally,  the  interlobular  septa  are  con- 
tinuous with  the  denser  laj'ers  of  fibrous  tissue  which 
form  the  investment  of  the  lobes:  the  latter, in  their  turn, 
being  continuous  with  the  pleura  which  forms  the  serous 
investment  of  the  surface  of  the  whole  lung. 

The  larynx  is  a  special  modification  of  the  proximal  end 
of  the  trachea:  it  differs  in  its  histological  structure  from 
the  latter  in  the  following  particulars.  Its  epithelium  is 
in  most  portions  stratified,  columnar  and  ciljated,  as  in 
the  trachea :  that  of  the  txue  vocal  cords  and  of  the  epi- 
glottis and  a  portion  of  the  intervening  surface  is  strati- 
fied squamous:  in  that  covering  the  under  surface  of  the 
epiglottis  numerous  taste  buds  are  imbedded.  The  mucosa 
differs  from  that  of  the  trachea,  chiefly  in  the  greater 
amount  of  adenoid  tissue,  in  places  resembling  the  phar- 
ynx in  this  respect :  the  elastic  fibres  which  run  through- 
out its  extent  are  greatly  increased  in  number  in  the  vocal 


200  PART    II.      HISTOLOGICAL   ANATOMY. 

cords,  where  they  constitute  the  chief  part  of  the  mucosa. 
The  principal  cartilages  are  hyaline,  like  those  of  the 
trachea:  those  of  Santorini,  of  Wrisberg  and  of  Luschka 
are  reticular,  as  is  the  epiglottis.  The  proper  muscles  of 
the  larynx  are  composed  of  striped  fibres,  resembling  in 
this  respect  the  musculosa  of  the  upper  portion  of  the 
oesophagus. 

The  course  and  arrangement  of  the  blood  vessels  of  the 
lung  are  subjects  rather  for  anatomical  than  histological 
discussion.  It  should  be  borne  in  mind,  however,  in  study- 
ing sections  of  that  organ,  that  the  lung  has  a  double 
blood  supply :  the  branches  of  the  pulmonary  artery  ac- 
companying the  bronchial  tubes  to  their  common  destina- 
tion in  the  lobules  of  the  lung;  while  the  smaller  bronchial 
arteries,  derived  from  the  aorta,  are  distributed  to  the 
walls  of  the  air  passages  themselves  and  to  the  surround- 
ing structures,  as  vessels  of  nutrient  supply.  A  similar  re- 
lation subsists  between  the  bronchial  veins,  which  empty 
into  the  ascending  vena  cava,  and  the  branches  of  the 
pulmonary  veins. 

The  lymphatics  of  the  lung  are  divisible  in  a^somewhat 
similar  manner  into  two  groups,  those  associated  with  the 
bronchial  tubes,  and  those  associated  with  the  air  sacs. 
The  former,  or  bronchial  lymphatics,  have  their  origin  in 
the  lymph  spaces  of  the  bronchial  mucosa,  forming  a  plexus 
in  the  submucosa  which  empties  into  larger  trunks  lead- 
ing to  the  root  of  the  lung.  The  second  group  is  compound 
of  two  sets,  the  deep  or  vascular  lymphatics,  having 
their  origin  in  the  connective  tissue  of  most  of  the  lobules 
of  the  lung,  and  the  superficial  or,  as  they  are  also  termed, 


CHAPTER    XVI.      RESPIRATORY   APPARATUS.  201 

the  subpleural  lymphatics,  which  arise  in  the  vicinity  of 
the  lobules  near  the  surface  and  enter  into  the  plexus  which 
underlies  the  pleura:  the  latter  communicates  with  the 
thoracic  cavity  bv  means  of  occasional  stomata.  The 
deep  and  superficial  plexus  alike  empty  into  trunks  which 
lead  to  the  root  of  the  lung,  there  to  enter,  in  company 
with  those  from  the  bronchial  lymphatics,  into  the  bron- 
chial lymph  nodes. 

It  is  a  common  custom  to  speak  of  the  lung  as  essentially 
similar  in  structure  to  a  gland,  the  alveoli  beingcompared 
with  the  similarly  named  divisions  of  the  latter,  theinfun- 
dibula  and  bronchioles  to  the  ductules  and  intralobular 
ducts,  and  the  bronchial  tubes  to  the  larger  ducts.  Com- 
parative anatom}-^  shows  that  the  primary  condition  of 
the  lung  is  that  of  a  large  sac-like  c»utgrowth  of  the  ali- 
mentary canal,  its  unquestionable  homologue  in  the  fish- 
like vertebrates  being  the  swim-bladder.  In  some  of  these 
forms  it  undergoes  more  or  less  subdivision  b\'  foldings  of 
its  inner  surface,  and  assumes  something  of  a  respiratory 
function;  its  connection  with  the  alimentary  canal  remain- 
ing simple  and  entirely  membranous:  in  the  lower  air- 
breathing  vertebrates  the  area  of  respiratory  surface  is 
increased  to  a  limited  extent  only  by  peripheral  sacculation, 
while  a  rudimentary  trachea  and  larynx  appears:  it  is 
only  in  the  higher  reptiles,  the  birds,  and  the  mammals, 
that  the  lung  assumes  the  compact  and  spongy  structure 
due  to  compound  sacculation  and  associated  with  the  pres- 
ence of  a  well  developed  system  of  bronchial  tubes.  Its 
origin  is  therefore  seen  to  be  the  reverse  of  that  of  a  large 


202  PART   II.      HISTOLOGICAL   ANATOMY. 

gland,  which  is  developed  by  an  increase  in  the  number  of 
origin alh'  small  alveoli. 

While  the  respiratory  tract  is  to  be  regarded  as  a  sub- 
divided saccular  outgrowth  of  the  alimentarj'  canal,  we 
must  not  expect  to  be  able  to  recognize  essential  similari- 
ty of  detailed  structure  in  organs  differing  markedly  in 
function :  a  general  comparison  may,  however,  aid  in 
understanding  and  remembering  the  character  and  ar- 
rangement of  the  tissues  present.  The  epithelium  is,  of 
course,  continuous  throughout.  The  mucous  membrane 
of  the  alimentary  canal,  with  its  varying  amount  of  ade- 
noid tissue  and  its  subjacent  muscular  layer,  is  represented 
in  the  air-passages  by  the  adeno-fibro-elastic  layer,  to 
which  in  the  bronchial  tubes  amuscularismucosaeis  added. 
The  submucosa  of  one  is  continued  into  that  of  the  other 
without  essential  modification.  The  outer  layer  of  the 
larger  air-passages  may  best  be  regarded  as  corresponding 
to  the  fibrosa  of  the  pharynx  and  oesophagus,  reinforced 
by  the  tracheal  and  bronchial  cartilages.  The  musculosa 
of  the  alimentary  canal  is  represented  only  by  the  muscu- 
lar area  of  the  trachea  with  its  well  developed  transverse 
and  rudimentary  longitudinal  layer. 


CHAPTER    XVII.      URINARY   ORGANS.  203 


CHAPTER   XVII. 
THE  URINARY  ORGANS. 


The  urinary  organs  include  the  glandular  kidneys;  their 
ducts,  the  ureters;  the  bladder,  to  which  the  latter  lead; 
and  the  urethra,  through  which  the  contents  of  the  blad- 
der are  discharged.  The  urethra  of  the  female  is  strictly 
a  urinary  tract,  and  is  throughout  its  entire  extent  a  por- 
tion of  the  ventral  outgrowth  of  the  alimentary  canal 
from  a  part  of  which  the  bladder  is  eventually  formed :  it 
will  therefore  be  described  in  this  connection.  The  pro- 
static portion  of  the  male  urethra  includes  all  that  part  of 
the  tract  homologous  with  the  urethra  of  the  female;  its 
remaining  portion  is  a  common  channel  for  the  urine  and 
the  seminal  fluid,  and  is  formed  in  connection  with  struc- 
tures accessory  to  reproduction :  the  description  of  its 
whole  extent  will  therefore  be  deferred  until  it  can  be 
taken  up  in  connection  with  them  in  a  subsequent  chapter. 

If  a  kidney  be  cut  through  from  the  convex  surface  to  the 
hilum,  either  longitudinally  or  transversely,  the  surface 
of  the  cut  section  shows  to  the  unaided  eye  certain  feat- 
ures of  importance  to  the  study  of  its  histology.  The  solid 
portion  is  seen  to  be  curved  around  a  central  cavity,  the 
pelvis  of  the  kidney;  into  this  project  a  number  (varying 
with  the  direction  of  the  section)  of  conical  papillae,  each 
of  which  is  surrounded  by  a  cup-like  extension  of  the  pel- 


204  PART    II.      HISTOLOGICAL   ANATOMY. 

vie  cavit}^  termed  a  calyx.  The  solid  portion  itself  can  be 
readily  seen  to  be  divided  with  considerable  sharpness  into 
an  outer  region,  the  cortex,  forming  about  one  third  of 
the  depth,  and  an  inner,  the  medulla ;  the  latter  can  be 
again  distinctly  subdivided  into  the  boundary  layer  next 
the  cortex,  and  forming  about  one  fourth  of  the  entire 
depth,  and  the  papillary  portion,  v^hich  includes  the  re- 
mainder. The  papillary  portion  is  distinctly  but  uni- 
formly striated  :  in  the  boundary  layer  radial  tracts,  termed 
medullary  rays,  similar  in  appearance  to  those  of  the 
papillary  substance  alternate  with  tracts  characterized  by 
increased  transparency. 

The  entire  mass  of  radiating  tracts  extending  from  the 
apex  of  a  papilla  to  the  meeting  of  the  cortex  and  the 
boundary  layer  constitutes  what  is  known  as  a  pyramid 
of  Malpighi;  there  are  rarely  less  than  ten  or  more  than 
twent}^  of  them  in  a  single  human  kidney :  in  some  of  the 
lower  mammals  they  are  numerous,  as  in  man  ;  in  others 
there  is  a  single  papillary  ridge  which  projects  into  the  pel- 
vis along  its  length,  or  a  single  large  central  papilla:  kid- 
neys such  as  the  latter  are  termed  umpyramidal.  If  the 
pyramids  of  Malpighi  be  conceived  of  as  extended  across 
the  cortex,  the  kidney  would  be  divided  into  a  correspond- 
ing number  of  parts  commonly  called  lobules :  if,  how- 
ever, we  are  to  compare  these  glands  with  others  on  the 
basis  of  the  arrangement  of  their  ducts,  these  regions  will 
with  greater  propriety  be  termed  lobes:  in  the  human  kid- 
ney they  are  not  structurally  separated,  but  in  that  of  some 
mammals,  as  the  otter,  each  is  invested  by  a  capsule  of  its 
own,  and  is  but  slightly  attached  to  its  fellows :  a  condi" 


CHAPTER    XVII.       I'KINARV   ORGANS.  205 

tion  characteristic  of  the  human  kidney  during  foetal  life. 
In  all  multipyramidal  kidneys,  like  that  of  man,  the  corti- 
cal portion  of  each  lobe  extends  around  and  beyond  the 
base_oLthe  pyramid  toward  the  pelvis  of  the  kidney  :  these 
interpyramidal  masses,  composed  in  part  of  portions  of 
the  cortical  substance  of  the  adjacent  lobules,  and  in  part 
of  blood  vessels  whose  origin  and  course  will  be  presently 
described,  are  known  as  the  columns  of  Bertin. 

The  medullary  rays  seen  in  the  boundary  layer  are  less 
readily  traceable  in  the  cortex  :  they  are,  however,  contin- 
ued into  that  layer,  growing  smaller  as  they  approach  the 
outer  surface,  and  disappearing  altogether  before  reaching 
it.  These  prolongations  of  the  medullary  rays  are  known 
as  the  pyramids  of  Ferrein:  their  bases,  as  will  be  readily 
understood,  rest  on  the  bases  of  the  pyramids  of  Mal- 
pighi,  each  of  which  gives  rise  tg  a  large  number  of  them. 
The  intervening  portions  of  the  cortex  make  up  what  is 
known  as  the  labyrinth:  -a  definite  portion  thereof,  not 
marked  off  bj'  any  visible  separation,  is  structurally  con- 
tinuous with  each  pyramid  of  Ferrein.  Each  pyramid  with 
its  associated  portion  of  the  labyrinth,  and  its  continua- 
tion b}'  means  of  the  medullar}'  ray  to  the  apex  of  the 
p\'ramid  of  Malpighi,  may,  as  we  shall  see,  be  with  pro- 
priety regarded  as  corresponding  in  extent  to  a  lobule  of 
other  glands.  Scattered  throughout  the  labyrinth  are 
small  rounded  bodies,  the  Malpighian  corpuscles,  barely 
visible  to  the  naked  eye.  Beyond  the  cortex  maybe  readily 
seen  the  tough  fibrous  capsule  which  invests  the  whole 
gland. 

The  kidney,  like  the  liver,  is  characterized  by  the  posses- 


206  PART  II.     HISTOLOGICAL  ANATOMY, 

sion  of  a  specially  modified  blood  supply,  the  most  impor- 
tant features  of  which  are  visible  to  the  naked  eye  or  with 
a  low  power  of  the  microscope.  If  a  section  be  made  as 
above  indicated  through  a  well  injected  organ  the  follow- 
ing facts  may  be  noted.  The  renal  artery,  entering  at  the 
hilum,  divides  into  four  or  five  branches  which  traverse  the 
lining  of  the  pelvis,  their  subdivisions  entering  the  columns 
of  Bertin.  As  they  pass  along  the  latter  they  give  off  twigs 
to  the  cortical  substance  present,  and  on  reaching  the  level 
of  the  bases  of  the  pyramids  of  Malpighi  give  rise  to  an 
arched  plexus  through  whose  meshes  the  pyramids  of  Fer- 
rein  pass  to  the  cortex.  From  the  arcuate  arteries  of  this 
region  branches  are  given  off  which  run  to  the  surface  of 
the  kidney,  known  as  the  radiate  or  (from  their  position 
between  the  cortical  areas  surrounding  the  pyramids  of 
Ferrein)  the  interlobular  arteries:  from  these  are  given 
off  twigs  which  supply  the  cortical  substance  in  a  manner 
presently  to  be  described :  their  extremities  terminate  in 
the  capillary  network  of  the  capsule  of  the  kidney.  From 
the  arcuate  arteries  also  arise  near  the  point  of  origin  of  the 
interlobular  branches  slender  vessels,  the  arteriae  rectae, 
which  supply  the  medullary  portion,  passing  directly 
toward  the  apex  of  the  papilla. 

There  may  also  be  seen  in  the  cortical  portion  of  the  kid- 
ney, and  receiving  twigs  which  proceed  from  its  substance, 
radiate  or  interlobular  veins  situated  in  close  proximit}^ 
to  the  arteries  of  that  name.  They  arise  just  beneath  the 
the  capsule  by  small  vessels  having  a  stellate  arrangement 
(the  stellules  of  Verheyen),  and  traverse  the  cortex  to  enter 
into  arcuate  veins  in  the  main  similarly  disposed  to  the 


CHAPER    XVII.      IRINARY    ORGANS.  207 

arteries  of  the  boundary  region ;  they  resemble  the  latter 
in  receiving  vena  rectae  from  the  medullary  portion,  and 
unite  to  form  trunks  which  pass  by  way  of  the  columns  of 
Bertin  to  traverse  the  lining  of  the  pelvis  and  come  together 
at  or  near  the  hilum  to  form  the  renal  vein.  The  vasa 
recta  pass  toward  the  papillae  in  groups  which  alternate 
in  the  boundary  layer  with  the  medullary  rays,  forming 
the  tracts  of  greater  transparency  above  mentioned. 

The  kidney  is  a  compound  tubular  gland  made  up  of 
lobes  and  lobules  which,  as  We  have  seen,  are  not  sharph- 
defined  from  each  other  by  fibrous  septa.  Its  component 
tubules  differ  in  a  marked  degree  from  those  of  any  other 
gland  in  the  definiteness  of  their  course;  the  variation  in 
size  and  in  the  character  of  the  epithelium  of  the  different 
regions  of  each ;  and  in  the  peculiar  relations  which  they 
sustain  to  their  blood  supply.  With  the  exception  of  the 
blood-vessels  and  their  accompanying  lymphatics  and 
nerves,  together  with  the  small  amount  of  connective  tis- 
sue in  which  these  are  imbedded,  and  of  a  very  scanty  inter- 
stitial tissue,  the  whole  substance  of  the  kidney,  both  cor- 
tical and  medullary,  is  made  up  of  these  tubules. 

Each  uriniferous  tubule  is  made  up  throughout  its 
whole  extent  of  a  homogeneous  basement  membrane  lined 
with  a  single  layei  of  epithelium.  It  has  its  origin  in  a 
Malpighian  bod\',  the  extremity  of  the  tubule  being  there 
expanded  to  form  a  thin  walled  sac  whose  distal  portion 
is  inverted  into  the  proximal,  forming  a  capsule  of  Bow- 
man: into  the  cavity  of  invagination  thus  formed  is  thrust 
a  spheroidal  mass  of  capillaries  termed  a  glomerulus:  its 


208  PART   n.     HISTOLOGICAL   ANATOMY. 

relations  will  be  described  later.  The  capsule  is  lined 
throughout  with  a  sirnple^quamous  epithelium ;  and  with 
the  accompanying  glomerulus  makes  up  the  Malpighian 
bod\'.  The  tubule  leaves  the  capsule  at  a  point  opposite 
the  inversion  for  the  glomerulus  bv  a  constricted  neck 
lined  with  small  cuboidaljoells,  expanding  at  once  to 
form  the  proximal  convoluted  tubule :  this  is  Hned  with 
cuboidal  cells,  whose  outhnes  are  irregular  and  interlock 
in  such  manner  as  to  render  it  almost  impossible  to 
distinguish  the  boundaries  of  adjacent  cells  when  seen 
in  longitudinal  section;  the  portion  qf^thecell  next  the 
b§S£m£Dt  membrane  is  vertically  striated,  and  is,  under 
the  influence  of  certain  reagents,  separable  into  rod- 
like processes ;  the  free  surface  bears  cilia-like  processes 
projecting  into  the  lumen.  The  tubule  passes  by  sweep- 
ing curves  toward  the  p\'ramid  of  Ferrein :  as  it  enters  it 
it  turns  toward  the  boundary  layer  in  wavy  curves  as  the 
spiral  tubule  of  Schachowa :  its  structure  is  essentially 
like  that  of  the  convoluted  tubule,  the  epithelium  being 
distinctly  "rodded." 

Entering  the  boundary  layer,  the  tubule  suddenly  be- 
comes much  smaller  and  nearly  straight  in  its  course 
toward  the  papilla :  it  is  here  known  as  the  descending 
tubule  of  Henle:  this  is  the  smallest  portion  of  the  whole 
tubule;  its  epithelium  has  the  formof  plate-like  cells,  whose 
nuclei  cause  a  central  thickening  which  projects  into  the 
conspicuous  lumen,  making  its  course  apparently  irregu- 
lar. Shortly  after  reaching  the  papillary  portion  the  tubule 
bends  upward,  forming  Henle's  loop,  the  diameter  in- 
creases slightly,  and   the  epithelium  assumes  the  form  of 


CHAPTER    XVn.      I'KINAKY   ORGAN'S.  209 

low  poU'hedral  cells,  greatly  reducin<^  the  lumen.  From 
the  loop  the  ascending  tubule  of  Henle  passes  upward 
through  the  boundary  layer  without  essential  modifica- 
tion, save  a  slight  increase  in  diameter,  as  a  straight  or 
slightly  wavy  tubule. 

The  ascending  tubule  enters  the  cortex  and  travels  for  a 
longer  or  shorter  distance  in  the  pyramid  of  Ferrein,  finally 
bending  abiniptly  to  enter  the  labyrinth  as  an  irregular 
tubule,  whose  size  and  the  position  of  whose  lumen  varies 
greatlv  in  accordance  with  irregular  variations  in  the 
height  of  the  striated  epithelium,  the  lumen  remaining  mi- 
nute throughout  its  whole  extent :  it  pursues  a  zigzagcourse 
toward  the  Malpighian  body  whence  the  tubule  arose.  In 
the  vicinity  of  the  latter  it  resumes  a  nearly  uniform  dia- 
meter and  exchanges  its  angular  course  for  regular  curves: 
it  is  now  known  as  the  distal  convoluted  tubule :  in  size, 
form,  and  structure,  it  is  identical  with  the  proximal.  Like 
the  latter,  its  course  tends  toward  the  pj'ramid  of  Ferrein; 
as  it  proceeds,  it  narrows  into  a  junctional  tubule,  or,  as 
it  is  also  called,  an  arched  collecting  tubule,  with  low 
cuboidal  epithelium  and  a  relativeh'  large  lumen.  It  passes 
toward  the  axis  of  the  p^'ramid  to  enter  a  straight  col- 
lecting tubule  somewhat  larger,  but  otherwise  similar  in 
structure,  which  may  also  receive  other  junctional  tubules 
from  point  to  point  as  it  passes  toward  the  boundary 
layer. 

The  straight  collecting  tubule  passes  along  the  medul- 
lary ray  through  the  boundary  layer  unchanged  in  form 
or  size :  after  it  enters  the  papillary  portion  it  joins  at  an 
acute  angle  with  similar  tubules,  the  resultant  tubules  be- 


210  PART   II.     HISTOLOGICAL   ANATOMY. 

coming  larger  with  each  union  :  the  largest  (and  terminal) 
tubes  so  formed,  known  as  the  ducts  of  Bellini,  are  lined 
with  a  simple  columnar  epithelium:  their  openings,  visible 
to  the  naked  eye,  are  scattered  over  the  apices  of  the  pa- 
pillae. 

The  interlobular  arteries,  as  has  been  previously  stated, 
pass  from  the  boundary  to  end  in  the  capillaries  of  thecor- 
tex.  Along  their  course  through  the  cortex  they  give  off 
on  every  side  short  arterioles  which  go  to  the  neighbor- 
ing Malpighian  bodies:  these,  which  are  known  as  the  vasa 
afiferentia,  on  reaching  these  bodies  give  rise  to  the  sphe- 
roidal masses  of  capillaries  called  glomeruli,  the  arteriole 
entering  the  glomerulus  at  a  point  opposite  the  neck  of 
the  capsule :  the  capillaries  unite  again  to  form  small  ves- 
sels, the  vasa  efferentia,  which  leave  the  glomeruli  where 
the  afferent  vessels  enter. 

The  efferent  vessels,  it  will  be  seen,  sustain  to  the  glo- 
meruli the  relation  of  veins :  they  do  not,  however,  proceed 
to  the  interlobular  veins,  but  divide  shortly  after  leaving 
the  glomeruli  to  give  rise  to  the  cortical  network  of  capil- 
laries, which  is  made  up  throughout  the  labyrinth  of 
short  irregular  meshes  interwoven  with  the  convoluted 
and  irregular  tubules,  throughout  the  pyramids  of  Fer- 
rein  of  long  meshes  running  in  the  direction  of  the  pyra- 
mid. The  capillary  network  is  not  distinguishable  by  any 
known  means  into  areas  corresponding  to  the  Malpighian 
bodies,  the  individual  tubules,  the  lobules,  or  even  the  lobes 
of  the  kidney,  being  apparently  continuous  throughout 
the  cortex  of  the  whole  kidney. 


CHAPER    XYII.      I  RINARV    ORGANS.  211 

Here  and  there  in  the  caj)illary  network  the  radicles  of 
small  veins  are  formed,  chiefly  in  the  vicinity  of  the  inter- 
lobular veins,  to  which  they  lead.  The  veins  in  question, 
arising  from  the  stellate  veins  just  beneath  the  capsule, 
gather  in  their  course  the  corticid  veinlets  and  pass  to  the 
arcuate  veins  of  the  plexus  lying  in  the  boundary.  They 
thus  form  the  channels  of  return  for  the  blood  which  leaves 
the  interlobular  arteries  by  way  of  the  vasa  afferentia, 
flowing,  as  should  be  noted,  through  two  sets  of  capil- 
laries, those  of  the  glomeruli  and  those  of  the  cortex,  as 
well  as  though  the  intervening  vasa  efferentia. 

The  vasa  recta,  as  has  been  stated,  leave  the  arteries  and 
veins  of  the  boundary  in  close  proximity  to  the  interlobu- 
lar vessels,  in  some  cases  springing  as  branches  from  their 
bases.  They  pass  in  the  transparent  striae  of  the  boundary 
layer  to  the  papilla,  the  arteries  being  resolved  along  the 
way  into  the  medullary  capillaries,  which  form  a  net- 
work with  greatly  elongated  meshes  running  in  the  direc- 
tion of  the  tubules  between  which  they  chiefly  lie :  in  the 
tip  of  the  papilla  the  meshes  are  much  shorter,  forming  a 
denser  network  about  the  ducts  of  Bellini.  The  capillaries 
are  gathered  up  into  the  radicles  of  the  veins,  which  lie  in 
close  proximity  to  the  arteries.  Occasionally  the  small 
vessels  which  arise  from  that  portion  of  the  cortical  net- 
work of  capillaries  nearest  the  boundary'  la\er,  instead 
of  entering  the  interlobular  veins,  pass  downward  into 
the  medulla:  these,  which  are  known  as  false  vasa  recta, 
sooner  or  later  divide  again  into  capillaries  which  enter 
into  the  medullary  network.  The  vasa  recta  divide  the 
medulla  into  regions  corresponding  to  the  lobules. 


212  PART    II.      HISTOLOGICAL   AXATOMY. 

The  capsule  of  the  kidne}-  is  a  thin  but  tough  fibrous 
membrane,  somewhat  lamellated  in  structure,  especiall}^ 
toward  the  outer  surface,  and  having  in  its  deep  portion  a 
scant}^ plexus  of  smooth  muscular  fibres.  Atthehilumthe 
capsule  is  continuous  with  the  fibrosa  of  the  ureter.  Its  sur- 
face is  invested  with  a  small  amount  of  adventitious  areo- 
lar tissue  which  in  some  cases  contains  numerous  fat 
lobules,  and  by  which  it  is  connected  ventrally  with  the 
fibrous  layer  ofthe  peritoneum,  and  dorsally  with  the  fascia 
of  the  adjacent  muscles.  A  plexus  of  lymphatics  has 
been  described  in  the  capsule  whose  channels  connect  with 
spaces  in  the  superficial  portion  of  the  cortex :  it  unites  at 
the  hilum  with  lymphatics  which  accompany  the  blood 
vessels  along  and  from  the  medullary  rays. 

The  lining  which  invests  the  pelvis  of  the  kidney  and  its 
prolongations,  the  calyces,  may  be  regarded  as  the  con- 
tinuation of  the  inner  coats  of  the  ureter,  and  may,  like 
the  wall  of  the  latter,  be  regarded  as  consisting  of  an  epi- 
thelial and  a  musculo-skeletal  layer.  The  epithelium  is  of 
the  transitional  type,  which  is  found  only  upon  urinary 
surfaces :  its  peculiarities  will  be  discussed  in  connection 
with  the  bladder,  in  whose  structure  they  are  most 
readily  demonstrable.  Immediately  beneath  is  a  thin  but 
dense  fibrous  membrane  which,  with  the  epithelium,  makes 
up  the  mucosa  of  the  pelvis.  This  constitutes  the  sole  in- 
vestment of  the  papillae,  the  two  layers  ofthe  mucosa  be- 
ing continuous  with  the  epithelium  and  basement  mem- 
brane respectively  of  the  uriniferous  tubules  at  the  mouths 
of  the  ducts  of  Bellini.  As  we  pass  along  the  surface  of 
the  calyx,   however,   the  mucosa  begins  to  be  separated 


CHAPTER    XYII.      URINARY   ORGANS.  213 

from  the  surface  of  the  columns  of  Berlin  by  scattered 
muscular  fibres,  which  increase  in  quantity  as  we  approach 
the  margin  of  the  fold  which  surrounds  the  tip  of  the  pa- 
pilla :  here  a  well  defined  muscular  ringis  formed,  which 
has  been  compared  to  the  transverse  layer  of  the  muscu- 
losa  ;  it  may  be  regarded  as  in  a  certain  sense  a  sphincter 
of  the  papilla.  The  general  surface  of  the  pelvis  is  lined 
by  the  mucosa  and  the  muscular  layer,  there  being  present 
between  the  two  an  inconspicuous  submucosa  of  areolar 
tissue,  which  also  contains  a  small  amount  of  adenoid 
tissue  and  scattered  mucous  glands. 

The  ureter,  as  has  been  stated,  consists  of  an  epithelial 
and  a  musculo-skeletal  layer;  the  components  of  the  latter 
being  so  distributed  as  to  form  with  the  epithelium  a  mu- 
cosa, a  submucosa,  a  musculosa,  and  a  fibrosa.  The  mu- 
cosa  is  a  continuation  of  that  just  described  as  lining  the  ;^./tv*i:^ 
pelvis  of  the  kidney,  with  which  it  agrees  in  structure  in 
every  essential  respect :  it  is  relatively  greater  in  quantity, 
being  thrown,  like  that  of  the  oesophagus,  into  longitudi- 
nal folds.  The  submucosa  is  correspondingly  increased  in 
quantity  :  as  in  the  bladder,  it  contains  a  small  amount  of 
diffuse  adenoid  tissue,  scattered  nodules  having  also  been 
described,  as  have  occasional  mucous  glands.  The  muscu- 
losa shows  throughout  the  whole  length  of  the  ureter  two 
well  defined  layers,  an  inner  longitudinal  and  an  outer 
transverse ;  and  along  the  lower  portion  traces  of  a  third 
layer,  external  to  the  circular,  are  found,  in  the  form  of 
scattered  longitudinal  bundles.  Each  of  the  principal 
layers  contains  a  comparatively  large  amount  of  inter- 
stitial connective  tissue  as  compared  with   the  muscular 


./^- 


,Lm^<^fci/uiM-  '•    toHMltj^  1^  J^my\hiyL 


214  PART  II.     HISTOLOGICAL  ANATOMY, 

layers  of  the  alimentary  canal :  outwardly  this  is  continu- 
ous with  a  well  defined  but  not  very  dense  fibrosa. 

The  bladder  resembles  the  ureter  in  the  essential  histo- 
logical structure  of  its  wall.  The  epithelium  which  lines 
the  mucosa  resembles  that  of  the  pelvis  of  the  kidney  and 
of  the  ureter,  as  has  been  stated :  the  term  transitional 
generalW  applied  to.it,  has,  as  it  is  generally  defined,  little 
meaning;  it  serves,  however,  to  connote  certain  character- 
istics pertaining  to  epithelium  found  only  on  urinary' 
surfaces,  and  distinguishing  it  from  stratified  squamous 
epithelium,  with  which  it  is  often  compared.  The  most 
marked  pecularity  of  transitional  epithelium  is  the  power 
possessed  by  all  of  its  cells  (and  not  the  deeper  layers  only, 
as  is  the  case  with  stratified  squamous  epithelium)  of 
changing  and  regaining  its  form  in  connection  with  the 
stretching  or  relaxation  of  the  membrane  beneath :  this 
can  be  most  readily  seen  by  comparing  sections  of  the  col- 
lapsed bladder  and  of  one  distended  by  a  hardening  fluid: 
both  conditions  will  therefore  be  described. 

In  the  former  case  the  transitional  epithelium  is  seen  to 
consist  of  three  distinct  forms  of  cells  arranged  in  what 
may  perhaps  be  termed  as  many  layers,  though,  as  will  be 
seen,  the  boundaries  which  separate  them  are  not  strongly 
marked.  The  surface  is  invested  with  a  single  layer  of  cells 
which  are  almost  spheroidal  or  cuboidal  above  (though 
usuall}'  not  as  deep  as  they  are  wide),  the  upper  surface 
being  either  flattish  or  slightly  convex  when  the  mucosa  is 
neither  stretched  nor  pressed  together,  and  almost  hemi- 
spherical when  the  bladder  is  strongly  contracted:  the 
lower  surface  of  the  superficial  cells  is  sculptured  by  con- 


CHAPTER     XVII.      IKINAKV    ORirANS.  215 

cavities  which  fit  closely  the  surfaces  of  the  cells  of  the 
next  layer:  two  nuclei  are  sometimes  seen  in  a  single  cell, 
and  it  is  quite  possible  that  the  superficial  cells  still  retain 
the  power  of  cell-division.  The  next  layer  consists  of  large 
pear-shaped  cells,  their  rounded  ends  fitting  into  the  exca- 
vations upon  the  under  surfaces  of  the  superficial  cells,  and 
their  smaller  tapering  extremities  extending  to  the  base- 
ment membrane  beneath.  The  spaces  between  the  large 
ends  of  the  pyriform  cells  and  the  membrane  are  occupied 
by  the  constituents  of  the  third  layer,  which  is  one  or  two 
cells  deep,  and  consists  of  smallerclosely  packed  spheroidal 
or  pol3'hedral  cells. 

If  the  bladder  be  distended,  the  whole  epithelial  layer 
becomes  much  thinner:  the  superficial  cells  become  flat- 
tened, approaching  squamous  cells  in  form;  they  never, 
however,  loose  their  characteristic  sculpturing:  the  pyri- 
form cells  become  greatly  shortened :  and  the  spheroidal 
cells  correspondingly  flattened.  A  knowledge  of  the  ap- 
pearance of  the  elements  in  both  these  conditions  is 
highly  important  on  account  of  the  frequent  appearance 
of  bits  of  epithelium  in  morbid  urine.  It  should  be  remem- 
bered also,  that  these  changes  doubtless  take  place  regu- 
larly with  the  daily  periodic  changes  in  the  state  of  the 
bladder.  It  is  well,  too,  to  note  the  fact  that  the  cells 
which  compose  transitional  epithelium  are  alwa3's  closely 
in  contact,  whatever  changes  of  form  they  may  undergo : 
and  also  that  this  epithelium  has  a  remarkable  power  of 
resisting  diffusion  into  the  blood  vessels  beneath  of  the 
soluble  constituents  of  the  urine. 

The  mucous  membrane  of  the  bladder  is  much  thicker 


216  PART    II.      HISTOLOGICAL  ANATOMY. 

and  firmer  than  that  of  the  ureter,  and  the  transition  to 
the  submucosa  is  much  more  abrupt.  As  in  the  ureter, 
a  muscularis  mucosae  is  w^anting,  while  a  small  amount 
of  adenoid  tissue  is  present.  The  Submucosa  contains 
numerous  elastic  fibres,  and  occasional  mucous  glands, 
particularly  toward  the  base. 

The  musculosa  consists  throughout  of  smooth  muscular 
fibres  whose  bundles  are  in  a  general  way  (and  particu- 
larly at  the  equator  of  the  bladder)  arranged  in  three 
layers,  a  middle  circular  and  an  outer  and  inner  longi- 
tudinal :  the  bundles  are,  however,  quite  irregularly  dis- 
posed, and  there  is  much  interstitial  connective  tissue :  it 
is,  therefore,  not  always  easy  to  recognize  the  layers.  At 
the  base  of  the  bladder  the  circular  laj^er  is  increased  in 
quantity  to  form  the  internal  sphincter.  The  interstitial 
connective  tissue  is  continuous  externally  with  a  rather 
loosely  woven  fibrosa,  which  is  over  a  part  of  the  outer 
surface  of  the  bladder  invested  with  serous  endothelium. 

The  female  urethra  continues  the  wall  of  the  bladder 
to  the  mucous  membrane  of  the  vestibule.  In  its  course 
the  epithelium  passes  from  transitional  to  stratified  squa- 
mous, the  remainder  of  the  mucosa  undergoing  no  impor- 
tant change.  The  submucosa  contains  numerous  elastic 
fibres  and  near  the  bladder  a  number  of  mucous  glands : 
its  deeper  layer  is  highly  vascular.  The  musculosa  con- 
sists of  an  inner  circular  and  an  outer  longitudinal  laj'er 
of  smooth  fibres,  and  there  is  no  well  defined  fibrosa. 


CHAPTER    XVIII.      REPRODICTIVE   ORGANS.  217 


CHAPTER  ZVIII. 
THE  MALE  REPRODUCTIVE  ORGANS. 


The  male  and  female  reproductive  bodies,  or  gonads, 
are  in  their  orijj^in,  mode  of  development,  and  primarv 
position  essentially  similar  bodies.  The  reproductive  ele- 
ments produced  in  them  differ  so  widely,  however,  in  their 
activities  as  to  call  for  widely  differing  mechanisms  for 
their  discharge:  mechanisms  which  are  nevertheless  de- 
rived from  the  modification  of  intimately  allied  structures, 
whose  development  is  alwa\'s  closel}-^  associated  with  that 
of  the  urinarv  apparatus.  For  this  reason  the  descrip- 
tion of  the  early  stages  of  the  latter  will  be  deferred  until 
the  completion  of  this  and  the  subsequent  chapter. 

The  male  gonads,  or  testes,  each  with  its  associated 
epididymis,  are  in  the  adult  human  subject  suspended 
by  the  spermatic  cords  in  saccular  folds  of  the  skin  conflu- 
ent below  the  penis  and  forming  the  scrotum.  The  latter 
is  formed,  as  will  be  more  fully  described  later,  by  the  push- 
ing down  of  the  abdominal  wall  on  either  side  of  the  base 
of  the  penis :  its  structure  therefore  conforms  to  that  of' 
the  body  wall,  subject,  however,  to  special  modifications 
in  each  of  its  constituent  layers. 

The  skin  of  the  scrotum  is  thin,  corrugated,  rich  in 
brownish  pigment  and  in  sebaceous  glands,  and  has  scat- 


218  PART   II.      HISTOLOGICAL  ANATOMY. 

tered  over  its  surface  flattened  curlinghairs  with  conspicu- 
ous bulbs.  The  superficial  fascia  of  the  groin  and  adjacent 
parts  is  continued  into  the  scrotum  to  form  a  character- 
istic tunic,  the  dartOS,  which  is  best  developed  in  the  fore- 
part of  the  scrotum :  it  is  quite  highly  vascular,  and  con- 
tains numerous  smooth  muscular  fibres ;  it  is  consequently 
of  a  reddish-brown  color.  The  right  and  left  dartos  tunics 
unite  in  the  mid-plane  to  form  a  partition,  the  septum 
scroti.  The  corrugation  of  the  skin  of  the  scrotum  is 
caused  by  the  constriction  of  the  muscular  fibres  of  the 
dartos. 

Immediately  internal  to  the  dartos  is  a  denser  and  firmer 
fibrous  layer,  thin  and  transparent,  known  as  the  sper- 
matic fascia:  it  is  derived  from  the  tendon  of  the  external 
oblique  muscle  of  the  abdominal  wall.  Within  and  closeU' 
associated  with  the  spermatic  fascia  is  a  layer  of  areolar 
tissue  which  contains  numerous  bundles  of  striped  muscu- 
lar fibres  variously  disposed,  constituting  a  more  or  less 
continuous  muscular  layer  termed  the  cremaster,  and 
formed  from  an  extension  of  the  external  oblique  muscle. 
Still  farther  within,  the  fascia  transversalis  is  continued 
in  the  scrotal  wall  by  the  infundibuliform  fascia,  a  fib- 
rous layer  separated  from  the  preceding  by  loose  areolar 
tissue  and  immediately  underlying  the  parietal  portion  of 
the  tunica  vaginalis,  a  serous  membrane  derived  from 
the  peritoneum,  which  lines  the  scrotum  and  is  reflected 
upon  the  spermatic  cord  and  testis. 

The  successive  la^^ers  characteristic  of  the  scrotal  wall 
are,  therefore,  from  without  inwards,  the  skin,  the  dartos 
tunic,  the  spermatic  fascia,  the  cremaster  muscle,  the  in- 


CHAPTER    XVIII.      REPRODUCTIVE  ORGANS.  219 

fundibuliform  fascia,  and  the  tunica  vaginalis.  Of  these 
the  (lartos  forms  the  basis  of  the  median  septum,  while 
those  internal  to  it  invest  the  cavities  which  are  separated 
thereby. 

The  spermatic  cord  of  either  side  is  composed  of  the  ves- 
sels and  nerves  of  the  testis  together  with  the  duct  of  dis- 
charge, known  as  the  vas  deferens :  they  are  imbedded  in 
areolar  tissue  and  surrounded  by  the  continuations  of  the 
coverings  of  the  testis.  The  structure  of  the  duct  will  be 
described  later. 

The  visceral  portion  of  the  tunica  vaginalis  invests  the 
surface  of  the  testis  except  its  posterior  border,  along 
w^hich  it  is  reflected  to  become  continuous  with  the  parie- 
tal portion :  it  is  frequently  termed  the  tunica  adnata. 
Beneath  it  lies  the  proper  capsule  of  the  testis,  the  tunica 
albuginea,  a  dense  white  fibrous  layer  of  considerable 
thickness:  along  the  posterior  margin  this  is  continued 
into  the  interior  for  some  distance  as  a  wedge-shaped  fib- 
rous reticulum  known  as  the  mediastinum  testis,  and 
also  as  the  corpus  Highmori:  from  the  mediastinum  ra- 
diate stout  straight  bands  of  fibrous  tissue,  the  septa  or 
trabeculae,  w^hich  unite  with  the  peripheral  albuginea, 
thus  imperfectly  dividing  the  body  of  the  testis  into  a  num- 
ber of  irregularK'  p}  ramidal  lobules.  The  inner  stratum 
of  the  albuginea  is  quite  vascular,  and  is  sometimes  dis- 
tinguished as  the  tunica  vasculosa:  from  it  vascular 
trunks  pass  along  the  septa  and  form  the  blood  supply  of 
the  lobules. 


220  PART  II.      HISTOLOGICAL  ANATOMY. 

Each  lobule  consists  of  a  variable  number  of  seminifer- 
ous tubules  supported  by  delicate  lamellae  of  interstitial 
tissue  continuous  with  the  stout  fibrous  septa  as  well  as 
with  the  proper  membranes  of  the  tubules :  within  these 
lamellae  peculiar  epithelioid  cells  are  found  either  singly  or 
in  groups,  whose  origin  and  function  are  still  uncertain ; 
they  are  known  as  the  interstitial  cells  of  the  testis. 

Each  seminiferous  tubule  begins  near  the  periphery  of 
the  testis  with  an  irregularly  contorted  portion  extending 
from  its  rounded  extremity  nearly  to  the  mediastinum 
and  known  as  the  convoluted  tubule,  or  the  seminifer- 
ous tubule  in  the  strict  sense :  these  occasionally  branch 
near  their  free  extremities,  and  are  said  to  anastomose  in 
some  instances.  As  they  approach  the  mediastinum  they 
become  smaller  and  less  irregular,  and  unite  to  form  the 
straight  or  conducting  tubules,  which  lie  in  the  apices  of 
the  pyramidal  lobules.  On  entering  the  reticular  medias- 
tinum the  latter  anastomose  freely  to  form  a  network  of 
tubules  of  variable  size,  forming  the  rete  testis,  whose 
meshes  correspond  with  the  spaces  of  the  mediastinum. 

The  membrana  propria  of  the  convoluted  tubules  con- 
sists of  several  lamellae  of  endothelioid  cells  with  flattened 
oval  nuclei.  Upon  this  membrane  rests  an  epithelial  layer 
several  cells  deep,  from  whose  inner  strata  are  derived  the 
male  reproductive  elements  or  spermatozoa.  The  base- 
ment membrane  of  the  straight  tubules  is  a  continuation 
of  that  of  the  convoluted :  it  is  lined  with  a  single  layer  of 
cuboidal  epithelium.  In  the  rete  testis  the  membrane  be- 
comes continuous  with  the  reticular  framework  of  the 
mediastinum,  and  can  no  longer  be  distinguished:    the 


CHAPTKR    XVIII.      REPRODUCTIVE   ORGANS.  221 

channels  are  lined  with  a  single  layer  of  flattened  epithe- 
lialjcells. 

The  arrano^enient  of  the  epithelial  cells  of  the  convoluted 
tubules,  and  the  changes  which  they  undergo  in  the  pro- 
cess of  forming  the  spermatozoa,  which  is  known  as  sper- 
matogenesis, have  been  the  subject  of  much  controversy 
and  cannot  vet  be  said  to  be  fully  understood.  Different 
methods  have  been  described  with  positiveness  not  only 
in  difl'erent  classes  of  animals,  but  in  different  orders  of 
the  mammals,  and  even  in  different  members  of  the  same 
order.  Our  knowledge  of  the  facts  in  the  human  subject 
are,  as  in  most  cases,  less  perfect  than  of  those  in  the  lower 
animals.  In  addition,  the  same  terms  have  been  applied 
bv  different  writers  to  what  are  certainly  different  stages. 
The  following  account  is  based  on  that  given  by  Schaefer 
in  the  tenth  edition  of  Quain's  Anatomy. 

Next  to  the  basement  membrane  is  found  a  layer  of  cells 
most  of  which  are  cubical,  clear,  and  possessed  of  nuclei 
which  are  in  the  resting  or  network  phase :  here  and  there 
dividing  nuclei  are  seen  :  these  cells  ma}' be  termed  parietal 
cells  or  spermatogonia.  Scattered  here  and  there  among 
them  are  larger  cells  which  project  between  the  more  in- 
ternal layers:  these  are  known  as  sustentacular  cells: 
w^hen_much  elongated  and  joined  to  bundles  of  develop- 
ing spermatozoa  the}-  form  the  columns  of  Sertoli. 

Within  the  parietal  la\'er  is  seen  a  middle  layer  of  some- 
what larger  spheroidal  cells  whose  nuclei  show  various 
phases  of  division,  the  intermediate  or  spermatogenic 
cells.     The  layer  may  be  one,  two  or  more  cells  in  depth  ; 


222  PART  II.      HISTOLOGICAL  ANATOMY. 

its  constituents  are  probably  derived  primarily  from  the 
cells  of  the  parietal  layer,  but  increase  in  number  by  lat- 
eral divisions.  They  are  by  some  authors  termed  sper- 
matocysts. 

The  inner  layer  of  distinctly  cellular  elements  consists  of 
smaller  and  more  numerous  cells  derived  from  the  inter- 
mediate layer,  the  spermatoblasts  (or  spermatids  of  some 
writers);  these  are  probably  directly  transformed  into  the 
spermatozoa.  When  first  formed  they  compose  a  layer  of 
closely  packed  small  granular  cells:  they  subsequently  be- 
come more  and  more  elongated  vertically  and  collected 
into  small  groups,  each  of  which  becomes  connected  with 
one  of  the  sustentacular  cells  above  mentioned  to  form  a 
column  of  Sertoli:  later  the  bundle  of  spermatozoa  derived 
from  each  group  of  spermatoblasts  becomes  separated 
from  its  sustentacular  cell;  the  constituent  elements  are 
set  free,  and  accumulate  in  large  numbers  in  the  lumen  of 
the  tubule. 

The  mature  spermatozoon  consists  of  an  oval  and  flat- 
tened head,  about  four  and  a  half  micra  long,  two  to  three 
broad,  and  one  to  two  thick;  a  cylindrical  rniddle  part  or 
body  about  six  micra  long  and  less  than  one  in  diameter; 
and  a  tapering  cilia-like  tail  forty  to  fifty  micra  long. 
The  head  consists  chiefly  if  not  entirely  of  the  nucleus  of 
the  spermatoblast :  the  origin  of  the  body  and  the  tail  is 
not  so  certain.  In  the  lower  animals  the  form  and  struct- 
ure of  spermatozoa  vary  greatly. 

The  p3^raraidal  lobules  and  the  body  of  Highmore,  with 
their  investment,  the  tunica  albuginea,  compose  the  whole 
of  the  gonad  proper  or  testis  in  the  strict  sense.    Closely 


CHAPTER    XVIII.      REPRonrCTIVE    ORGANS.  223 

associated  therewith  and  often  regarded  as  a  portion 
thereof,  though  in  reality  the  beginning  of  the  efferent  appa- 
ratus, is  the  epididymis,  a  tubular  mass  attached  to  the 
testis  in  the  region  of  the  mediastinum.  Associated  with 
the  latter  are  certain  rudimentar\' organs  known  respec- 
tively as  the  hydatids  of  Morgagni,  lying  between  the 
head  of  the  epididymis  and  the  upper  end  of  the  testis, 
the  vas  aberrans,  attached  to  the  lower  end  of  the  epidid- 
ymis, and  the  organ  of  Giraldes,  found  near  the  base  of 
the  spermatic  cord. 

The  channels  of  the  rete  testis  anastomose  freely  through  - 
out  the  whole  of  the  mediastinum  or  body  of  Highmore. 
Those  of  the  upper  or  anterior  portion  open  into  the  effer- 
ent tubules,  also  known  as  vasa  efferentia,  from  twelve 
to  twenty  in  number,  which  penetrate  the  albuginea  and 
enter  the  epididymis,  of  which  they  form  a  part :  at  first 
straight,  they  soon  become  coiled  in  conical  masses,  the 
coni  vasculosi,  whose  bases  are  turned  away  from  the 
testis.  These  aggregated  conical  bodies,  together  with 
the  upper  portions  of  the  canal  of  the  epididymis,  into 
which  the  efferent  tubules  open,  form  the  globus  major, 
or  caput  epididymis.  Their  constituent  tuljules  are  as 
large  as  the  convoluted  tubules  of  the  testis:  they  are 
lined  with  a  cuboidal  or  short  columnar  ciliated  epitheli- 
um, beneath  which  is  a  well  defined  membrana  propria 
surrounded  b}'  a  thin  transverse  layer  of  smooth  mus- 
cular fibres. 

The  canal  of  the  epididymis,  exceedingh-  flexuous 
from  the  point  of  its  origin  in  the  extremities  of  the  up- 
permost vasa  efferentia,  becomes  disposed  in  numerous 


224  PART  II.      HISTOLOGICAL  ANATOMY. 

small  irregular  coils  in  the  middle  region  and  is  continued 
at  the  lower  portion  as  a  densely  convoluted  mass,  the 
globus  minor.  At  its  origin  it  is  twice  the  diameter 
of  the  vasa  efferentia:  lower  it  becomes  smaller,  and  is 
enlarged  again  in  the  globus  minor,  from  which  it  is  con- 
tinued as  the  beginning  of  the  spermatic  duct  or  vas  def- 
erens. It  is  lined  throughout  its  course  with  tall  col- 
umnar cells  (between  whose  bases  smaller  rounded  cells 
occur) :  these  cells  are  provided  with  unusually  long  cilia 
throughout  the  greater  portion  of  the  tube,  the  cilia  dis- 
appearing in  the  lower  portion.  Beneath  the  underlying 
membrane  is  a  thin  circular  layer  of  smooth  muscular 
fibres  continuous  with  that  of  the  vasa  efferentia,  which 
is  surrounded  by  a  thin  longitudinal  layer.  The  convolu- 
tions of  the  tube  are  bound  together  by  interstitial  areo- 
lar tissue  which  is  here  and  there  replaced  by  incomplete 
fibrous  septa  [w^hich  partially  divide  the  'epididymis  into 
irregular  lobules. 

The  hydatids  of  Morgagni  are  small  saccular  bodies 
lying  between  the  globus  major  and  the  upper  end  of  the 
testis.  One  of  these,  the  stalked  hydatid,  is  provided 
with  a  short  peduncle,  is  usually  present,  and  has  a  homo- 
logue  in  the  female  organs  of  reproduction.  The  others, 
the  sessile  hydatids,  are  variable  in  number  and  some- 
times wanting:  they  are  found  in  the  male  only.  Both 
stalked  and  sessile  hj^datids  are  lined  with  cuboidal  epi- 
thelium upon  which  cilia  have  been  described. 

The  vas  aberrans  is  a  long  narrow  blind  tube,  a  diverti- 


chaptp:r   XVIII.     rki»K()I)1'CTivk  okcans.  JJo 

culum  of  the  canal  of  the  epididymis,  which  arises  from 
the  latter  at  or  near  the  point  where  it  becomes  continu- 
ous with  the  vas  deferens.  Like  the  canal,  it  is  exceedingly 
tortuous  in  its  course,  forming  an  elongated  convoluted 
mass  which  extends  upward  among  the  vessels  of  the 
spenut'itic  cord.  It  resembles  the  vas  deferens  in  struct- 
ure. It  is  almost  invariably  present,  is  sometimes  branch- 
ed, and  in  some  cases  more  than  one  such  structure  oc- 
curs. 

The  organ  of  Giraldes,  also  called  the  paradidymis,  is  a 
small  body  found  on  the  front  of  the  spermatic  cord  just 
above  the  globus  major.  It  consists  of  several  discon- 
nected irregularly  branched  tubules  lined  with  columnar 
ciliated  epithelium.  Their  coiled  masses  form  irregular 
nodules  imbedded  in  the  connective  tissue  of  the  regions 
between  the  cord  and  the  epididj^mis. 

The  spermatic  duct,  or  vas  deferens,  is  the  continua- 
tion of  the  canal  of  the  epididymis.  It  is  considerably 
larger  and  more  complex  in  structure  than  the  canal, 
showing  like  many  of  the  tubular  structures  of  the  bod}', 
a  definite  mucosa,  submucosa,  and  musculosa.  The  mu- 
cosa consists  of  a  stout  membrane  sometimes  throw^n 
into  Ipngitudinal  folds,  and  bearing  a  non-ciliated  colum- 
nar epithelium  :  the  submucosa  is  composed  of  areolar  tis- 
sue somewhat  laminated  in  arrangement :  the  musculosa 
is  thick  and  yellowish  in  color  and  comprises  an  inner  cir- 
cular and  an  outer  longitudinal  layer  of  smooth  muscular 
fibres,  external  to  which  is  a  fibrous  adventitia.    At  the 


226  PART  II.      HISTOLOGICAL  ANATOMY. 

commencement  of  the  duct  there  is  also  a  longitudinal 
layer  of  muscular  fibres  internal  to  the  circular  layer. 

The  ampulla,  or  sacculated  enlargement  of  the  duct  sit- 
uated near  its  junction  with  the  seminal  vesicle,  resembles 
the  rest  of  the  duct  in  structure,  save  that  the  various 
coats  are  somewhat  thinner.  The  tubular  diverticula  of 
thevasa  deferentia  known  as  the  seminal  vesicles  are  like 
the  ampullae  in  structure.  The  spermatic  duct  or  vas  de- 
ferens of  either  side  unites  with  its  associated  seminal  vesi- 
cle to  form  the  ejaculatory  duct,  or  common  seminal 
duct,  which  completes  the  passage  way  from  the  seminif- 
erous tubules  of  the  testis  to  the  urethra :  in  this  region 
the  walls  of  the  tube  are  much  thinner  than  in  the  vas 
deferens  and  the  external  fibrous  adventitia  disappears 
as  the  duct  enters  the  substance  of  the  prostate  gland. 

The  male  urethra,  into  which  the  common  seminal  ducts 
empty  within  the  region  surrounded  by  the  prostate  gland, 
is  by  virtue  of  that  fact  divisible  into  two  distinct  regions, 
the  urinary  and  the  urino-genital :  it  is  the  former  alone 
that  corresponds  to  the  female  urethra.  By  its  anatomi- 
cal relations  it  is  also  divided  into  the  prostatic,  the  mem- 
branous, and  the  penial  urethra :  the  former  includes  the 
first  of  the  two  regions  above  mentioned  and  a  portion  of 
the  other.  Histologically  the  structure  differs  in  the  sev- 
eral regions  chiefly  in  the  character  of  the  epithelium,  in 
the  special  structures  found  in  the  mucosa  of  the  penial 
region,  and  in  the  composition  of  the  musculosa. 

The  prostatic  urethra  of  the  male  resembles  the  female 
urethra  in  being  essentially  a  continuation  of  the  wall  of 


CHAPTER    XVIII.      REPRODUCTIVE  ORGANS.  227 

the  bladder.  It  has  a  mucosa  lined  with  transitional  epi- 
thelium, which  rests  upon  a  membrane  rich  in  elastic  fibres. 
Beneath  is  a  highly  vascular  submucosa.  and  beyond  this 
a  musculosa  which  consists  of  an  inner  longitudinal  and 
outer  circular  layer  of  smooth  muscular  fibres.  In  front 
of  the  openings  of  the  common  seminal  ducts  the  epjthe- 
lium  passes  by  a  'gradual  modification  from  the  transi- 
tional to  a  somewhat  stratified  columnar  form.  The  -^ 
membranous  urethra  continues  the  structure  of  the  an- 
terior portion  of  the  prostatic,  and  in  addition  receives  a 
distinct  layer  of  striped  muscular  fibres  from  the  adjacent 
compressor  urethrae. 

In  the  penial  urethra  the  epithelium  is  composed  of  a 
single  layer  of  columnar  cells  except  at  the  fossa  navicu-  ^ 
laris,  where  it  passes  into  stratified  squamous  epithelium  if 
continuous  with  that  of  the  surface  of  the  glans.  Here 
and  there  the  mucosa  exhibits  irregular  depressions  of 
variable  size,  known  as  lacunae  Morgagni :  it  also  has 
connected  with  it  numerous  small  racemose  glands,  the 
glands  of  Littre,  which  are  also  sparingly  found  in  other 
portions  of  the  urethra;  they  are  lined  with  cuboidal  or 
low  columnar  glandular  cells. 

The  penis  consists  essentially  of  three  masses  of  what 
is  commonly  called  erectile  tissue,  the  right  and  left  cor- 
pora cavernosa,  and  the  median  inferior  corpus  spongi- 
osum ;  the  latter  being  traversed  throughout  its  length  by 
the  penial  urethra,  and  having  its  distal  portion  expanded 
to  form  the  glans  penis :  the  whole,  of  course,  invested  by 
the  somewhat  modified  integument.    The  form  and  rela- 


228  PART    II.      HISTOLOGICAL    ANATOMY. 

tions  of  these  bodies  are  matters  for  anatomical  discus- 
sion :  we  are  here  concerned  with  their  histological  struct- 
ure onh'. 

What  is  known  as  erectile  tissue  consists  primarily 
"sirapl}'  of  a  somewhat  circumscribed  collection  of  larger 
and  smaller  veins  w^hich  under  certain  circumstances  may 
become  distended  with  blood,  thus  causing  the  parts  in 
which  they  lie  to  expand."  Its  specialization  consists 
chiefly  in  the  enlargement  of  the  veins  to  form  irregular 
sinuses,  or  cavernae,  and  their  frequent  anastomosis  ;  the 
development  and  modification  of  the  circumscribing  skel- 
etal tissues ;  and  accessory  modifications  of  the  arteries 
of  supplv.  That  the  cavernous  sinuses  are  to  be  regarded 
as  veins  is  showm  not  only  b}'  the  striicture  of  their  walls, 
but  also  by  the  fact  that  the  blood  reaches  them  chiefly  if 
not  entirely  through  capillaries.  It  will  be  seen  from  w^hat 
has  just  been  said  that  erectile  tissue  is  not  constant  in 
structure  as  are  adenoid  and  other  compoutjd  tissues  that 
have  been  previoush^  described.  The  form  assumed  in 
each  locality  where  it  occurs  will  therefore  be  briefly 
stated. 

The  corpora  cavernosa  are  surrounded  and  united  by  a 
stout  fibrous  envelope,  termed,  like  that  of  the  testis,  the 
tunica  albuginea ;  it  consists  of  bundles  of  white  fibres 
chieflv  disposed  in  a  longitudinal  direction  and  mixed  with 
numerous  elastic  fibres:  within,  the  w^hite  fibres  are  chiefly 
circularly  disposed,  surrounding  each  of  the  two  corpora 
to  form  an  individual  sheath;  these  two  sheaths   are 


CHAPTER    XVIII.      REPRODUCTIVE   ORGANS.  229 

confluent  in  the  mid-plane  through  the  greater  part 
of  the  penis,  tbnning  a  septum  which  is  incomplete  by 
virtue  of  the  presence  of  slit-like  apertures  through  which 
the  erectile  tissue  becomes  continuous  from  side  to  side: 
these  ajiertures  are  most  numerous  in  the  anterior  part  of 
the  penis. 

From  the  fibrous  sheath  numerous  stout  trabeculae 
pass  inward  to  form  a  reticulum  whose  meshes  are  the 
cavernous  sinuses.  They  are  composed  chiefly  of  white 
fibrous  tissue,  with  more  or  fewer  elastic  fibres  inter- 
mingled, and  contain  in  addition  numerous  bundles  of 
smooth  muscular  fibres :  their  surfaces  are  lined  with  the 
vascular  endothelium  of  the  sinuses.  The  trabeculae  are 
stoutest  and  most  numerous  near  the  surface:  the  sinuses 
are  correspondingK'  largest  at  the  centre  of  the  bod\' ; 
thev  are  also  larger  near  the  extremity  of  the  penis  than 
at  the  base,  in  which  region  their  long  diameter  is  as  a 
rule  placed  transversely  to  the  penis.  The  small  arteries 
which  follow  the  trabeculae  in  many  cases  project  from 
their  surfaces  in  peculiarly  curled  and  coiled  loops;  they 
are  hence  known  as  helicine  arteries:  they  are  said  in 
some  cases  to  open  directly  into  the  sinuses,  but  such  a 
direct  communication  between  an  arter\'  and  a  modified 
vein  is  not  easily  demonstrated  under  conditions  which 
exclude  the  possibility  of  error. 

The  corpus  spongiosum  differs  from  the  corpora  caver- 
nosa histologically  in  the  thinness  and  increased  elasticity 
of  its  fibrous  tunic,  which  contains  so  much  elastic  tissue 
as  to  be  yellowish  in  color ;  in  the  smaller  size  and  greater 


230  PART   II.      HISTOLOGICAL   ANATOMY. 

uniformity  of  the  trabeculae;  and  in  the  lesser  amount  of 
muscular  tissue  which  they  contain.  The  venous  sinuses 
are  smaller  and  more  uniform,  forming  a  spongy  mass, 
from  which  the  name  is  derived  :  their  greatest  dimensions 
are  as  a  rule  longitudinally  disposed. 

In  the  glans  the  meshes  are  quite  small  and  uniform 
in  size :  the  erectile  tissue  passes  insensibly  into  the  lower 
strata  of  the  integument,  with  which  its  surface  is  invested. 
The  derma  is  thin  and  highly  vascular  over  the  surface 
of  the  glans,  and  the  epidermis  has  the  form  of  a  stratified 
squamous  epithelium  devoid  of  the  division  into  layers 
characteristic  of  the  cuticle  and  resembling  in  character 
that  found  on  the  oesophageal  mucous  membrane.  Glands 
arewanting,  except  upon  the  corona  and  the  cervix,  where 
modified  sebaceous  glands,  the  glands  of  Tyson,  are 
abundant.  Special  nerve  terminals,  the  so-called  genital 
corpuscles,  are  present,  as  are  Pacinian  bodies. 

The  skin  of  the  penis  is  quite  thin,  highly  elastic,  and 
very  movable,  and  contains  but  a  very  little  fat :  the  larger 
portion  is  devoid  of  hair  also:  as  it  passes  around  the 
free  margin  of  the  prepuce  it  changes  its  structure  and  the 
character  of  its  epithelium,  the  lining  of  the  prepuce,  like 
the  investment  of  the  glans,  having  the  appearance  of  a 
mucous  membrane.  At  the  base  it  passes  on  into  that  of 
the  pubes,  which  is  quite  thick,  beset  with  coarse  hairs, 
and  provided  with  a  dense  fatty  layer. 

The  urino-genital  tract  of  the  male  has  associated  with 
it  certain  glandular  bodies,  the  prostate  gland,  which  sur- 


CHAPTKR    XVIII.      REPRODUCTIVE    ORGANS.  231 

rounds  the  proximal  region  of  the  urethra,  and  the  glands 
of  Cowper,  paired  organs  opening  into  it  near  the  point 
where  it  enters  the  corpus  spongiosum.  Opening  into  it 
ventrally  in  close  proximity  to  the  apertures  of  the  ejacu- 
latory  ducts,  is  an  interesting  rudiment,  the  sinus  pocu- 
laris,  otherwise  known  as  the  uterus  masculinus.  These 
structures  will  next  be  described. 

The  prostate  Ts  a    glandular  body   which   differs  from 
most  organs  of  the  kind  in  the  fact  that  not  only   its  cap- 
suje  but  also  its  stroma  contains   a  ver}-  considerable 
amount  of  smooth  muscular  tissue.    This  muscular  tissue    / 
is  in  continuity  with  the  musculosa  of  the  urethra  and  of    J 
the  ejaculatory  ducts,  and  posteriorly  with  that  of  the     » 
bladder.     The  capsule  is  divisible  into  two  layers,  between 
which  is  found  the  prostatic  venous  plexus:  from  it  trabec- 
ulae  pass  inward  to  form  the  framework  of  the  gland,  con- 
'   sisting,  in  addition  to  the  smooth  muscular  tissue  already 
J  mentioned,  of  a  very  small  amount  of  white  fibrous  tis- 
sue and  a  larger  quantity  of  elastic  fibres.    The  alveoli  are  ) 
f  tubular,  frequently  quite  elongated  and  irregular  in  shape, 
I  their  walls  sometimes  showing  conspicuous  folds :  the  epi- 
thelium is  columnar  and  simple  save  that  frequently  small 
and  spheroidal  cells  are  found  at  the  base  of  the  columnar 
)  cells.    The  ducts,  which  are  numerous,  are  lined  with  col- 
umnar epithelium  which  changes  into  stratified  as  it  ap- 
1    proaches  their  openings  upon  the  urethra. 

Cowper's  glands  are  small  bodies  of  the  racemose  type, 
each  consisting  of  several  small  lobes.   Their  capsules  and 


232  PART    II.      HISTOLOGICAL    ANATOMY 

supporting  framework  resemble  those  of  the  prostate  to 
some  extent  in  the  presence  in  each  of  a  small  amount  of 
smooth  muscular  fibre :  a  well  defined  longitudinal  laj'-er 
of  smooth  muscular  fibres  is  also  present  in  the  wall  of 
the  principal  duct.  The  acini  resemble  those  of  a  mucous 
salivary  gland  in  form  and  in  the  general  appearance  of 
the  glandular  epithelium :  there  is  a  conspicuous  lumen, 
the  cells  are  pyramidal,  and  the  nuclei  are  situated  near 
the  base.  Nothing  resembling  the  crescents  or  demilunes 
of  the  mucous  glands  has  been  observed.  The  lobar  ducts 
are  lined  with  cuboidal  epithelium,  which  passes  into  col- 
umnar in  the  principal  ducts. 

The  sinus  pocularis  is  the  homologue  in  the  male  of  the 
vagina  and  uterus  of  the  female.  It  is  a  diverticulum  of 
the  prostatic  urethra  having  a  well-defined  muscular  wall 
and  a  mucosa  containing  a  number  of  short  tubular 
glands  which  resemble  the  uterine  glands  in  their  form 
and  structure. 


chai'Ti;k  XIX.     reproductive  organs.  23v 


CHAPTER  XIX. 
THE  FEMALE  REPRODUCTIVE  ORGANS. 


The  female  reproductive  apparatus  consists  of  the  fe- 
male gonads,  or  ovaries,  in  which  the  reproductive  ele- 
ments are  formed,  the  oviducts,  or  Fallopian  tubes,  by 
which  they  are  conveyed  from  the  ovaries;  the  uterus,  in 
which  they  are  received,  and  in  which  the  fertilized  ovum 
or  oosperm  develops  into  the  embryo ;  the  vagina,  by 
which  the  uterus  communicates  with  the  exterior,  and  the 
parts  composing  the  vulva,  w^hich  immediately  surrounds 
the  openingof  the  vagina.  As  in  the  case  of  the  male,  there 
are  also  present  certain  rudimentary  bodies,  chiefly  in  the 
vicinity  of  the  gonad. 

The  ovary,  like  the  testis,  is  an  organ  in  which  special- 
ized cells,  epithelial  in  their  origin,  are  matured  and  liber- 
ated :  the  sexual  elements  differ,  however,  in  an  antipodal 
manner  as  regards  their  size,  activity,  destination,  mode  of 
transportation  thereto,  and  mode  of  liberation :  there  is 
a  corresponding  difference  in  the  structure  of  the  organs 
which  produce  them,  the  ovary  having  nothing  of  that 
tubular  structure  seen  in  the  testis  and  giving  to  that 
body  a  close  resemblance  to  a  gland. 

The  framework  or  stroma  of  the  ovary  lacks  the  abund- 
ant white  fibrous  tissue  found  in  the  capsule  and  trabecu- 


234  PART   II.      HISTOLOGICAL   ANATOMY. 

lae  of  the  testis :  it  consists  chiefly  of  a  peculiar  form  of 
connective  tissue  characterized  by  the  presence  of  elon- 
gated nucleated  cells  which  are  frequently  spindle  shaped, 
and  b}^  the  scarcity  of  true  fibrous  tissue,  either  white  or 
elastic.  It  contains  numerous  smooth  muscular  fibres, 
which  are  most  abundant  in  the  deeper  portions.  Toward 
the  surface  the  stroma  becomes  more  dense,  forming  quite 
a  well-defined  superficial  layer,  to  which  the  name  of  the 
tunica  albuginea  has  been  given ;  it  lacks  the  firmness 
and  definiteness  of  the  layer  so  designated  in  the  testis. 
At  the  base  of  the  ovary  the  stroma  is  especially  rich  in 
blood  vessels :  the  region  occupied  by  them  is  known  as 
the  zona  vasculosa. 

The  region  between  the  albuginea  and  the  zona  vascu- 
losa constitutes  the  parenchyma  of  the  ovary.  It  is 
rather  indefinitely  divided  into  a  cortical  and  a  medullary 
portion  by  the  character  of  the  Graafian  follicles  contained 
in  it.  It  also  contains,  scattered  irregularly  through  it, 
groups  of  interstitial  cells  similar  to  those  found  in  the 
testis. 

The  surface  of  the  ovary  is  invested  with  a  layer  of  cells 
which  are  structurally  continuous  with  the  serous  endo- 
thelium of  the  peritoneum,  but  which  differ  therefrom  in 
form  and  function.  They  are  cuboidal  or  low  columnar 
in^ shape,  and  constitute  the  germinal  epithelium:  they 
have  no  proper  basement  membrane,  but  rest  directly  up- 
on the  tunica  albuginea.  Here  and  there  may  be  seen,  es- 
pecialh-^  in  the  embrj^o,  certain  cells  which  are  larger  and 
more  rounded  infoi'm  :  these  are  the  primitive  OVa.  These, 
during  foetal  life  and  possibly  in  childhood  sink  into  the 


CIIAPTKK    XIX.      RHI'KODrCTIVK   DKGANS.  235 

Stroma,  accompanied  by  tubular  or  spheroidal  nests  of 
epithelial  cells :  it  is  doubtful  whether  this  ever  takes  place 
in  the  adult :  it  is  also  at  present  a  matter  of  question 
whether  the  primitive  ova  increase  in  number  by  division 
after  they  have  passed  into  the  stroma,  or  whether  all  so 
situated  have  come  from  the  epithelium  of  the  surface  of 
the  ovary. 

The  cortical  region  of  the  parenchyma  is  crowded  with 
the  spheroidal  masses  of  cells  formed  in  the  manner  just 
described,  each  consisting  of  one  or  sometimes  two  primi- 
tive ova  surrounded  by  a  layer  of  epitheHal  cells.  These 
are  the  primitive  Graafian  follicles.  In  those  immediately 
beneath  the  albuginea  the  surrounding  layer  is  usually 
but  a  single  cell  deep:  at  first  flattened,  and  hardly  dis- 
tinguishable from  the  cells  of  the  adjacent  stroma,  its  cells 
soon  become  cuboidal  in  form.  At  the  same  time  the 
fibres  of  the  stroma  tend  to  assume  a  disposition  concen- 
tric to  the  follicle,  forming  the  beginning  of  the  theca 
folliculi. 

As  the  follicles  grow  older  they  tend  to  sink  deeper  into 
the  stroma  of  the  ovary;  the  cells  of  the  enveloping  layer 
at  the  same  time  proliferating,  and  the  layer  becoming 
several  cells  thick.  Shortly  afterward  the  ovum  leaves  its 
central  position  for  one  nearer  one  side  of  the  follicle,  usu- 
uallv  that  farthest  from  the  surface  of  the  ovary,  while 
a  cleavage  takes  place  in  the  cellular  layer  toward  the 
other  side,  the  space  formed  becoming  infiltrated  with  a 
clear  fluid,  the  liquor  folliculi.  The  follicles  now  rapidly 
increase  in  size,  at  the  same  time  sinking  into  the  medul- 
lary region,  where  they  are  seen  as  large  vesicles  filled 


236  PART    II.      HISTOLOGICAL    ANATOMY 

with  fluid,  invested  by  a  well-defined  theca,  and  lined  by  a 
layer  of  small  isodiametric  cells  of  irregular  form,  the 
layer  being  several  cells  deep :  it  is  now  known  as  the 
membrana  granulosa.  Attached  to  it  at  one  side  of  the 
follicle  is  the  rounded  heap  of  similar  cells  which  contains 
the  ovum :  it  is  termed  the  discus  or  cumulus  prolig- 
erus. 

The  follicles  still  increasing  in  size,  their  outer  wall  now 
tends  to  approach  the  surface  of  the  ovary,  the  fulh'  ma- 
tured follicle  finally  projecting  somewhat  from  the  surface, 
b}^  whose  rupture  the  contained  ovum  is  eventually  to  be 
liberated.  The  theca  is  now  well  defined  and  consists  of 
two  laj'ers,  an  inner  or  vascular,  and  an  outer  or  fibrous 
layer.  The  cells  of  the  membrana  granulosa  next  the 
theca  and  those  of  the  cumulus  next  the  ovum  are  dis- 
tinctly columnar  in  form.  The  most  conspicuous  as  well 
as  the  most  important  body  present  is  the  mature  ova- 
rian ovum.  This  is  a  spheroidal  body  now  much  larger  than 
the  primitive  ovum  from  whi&hit  was  developed,  although 
small  as  compared  with  the  ova  of  many  of  the  lower  ver- 
tebrates. It  is  in  the  human  subject  about  two-tenths  of 
a  millimetre  in  diameter.  It  is  invested  b}--  a  thick  cover- 
ing appearing  when  seen  with  microscopes  like  those  used 
by  the  earlier  observers  to  be  quite  clear :  as  its  optical  sec- 
tion forms  a  girdle  or  zone  of  considerable  breadth  about 
the  ovum  it  was  named  by  Von  Baer  the  zona  pellucida. 
Careful  examination  by  modern  instruments  and  meth- 
ods demonstrate  that  it  contains  innumerable  radial 
striae:  it  is  therefore  now  commonly  called  the  zona 
striata,  or  still  more  accurately  the  striated  membrane. 


ClIAPTEK     XIX.      KKI'UODrCTIVi:    ORCANS.  237 

Several  eminent  and  accurate  observers  have  described  a 
delicate  membrane,  which  they  call  the  vitelline  mem- 
brane, internal  to  the  layer  just  described  :  its  presence 
cannot  l^e  readily  demonstrated  with  certainty. 

Within  the  envelope  just  described  is  the  vitellus,  or 
volk,  wrongly  so  called,  since  it  does  not  correspond  to 
the  bod  v  so  termed  in  the  eggs  of  many  lower  vertebrates: 
it  is  a  homogenous  protoplasmic  mass,  semi-fluid  in  con- 
sistencv  and  highly  granular.  It  contains,  usually  in  an 
eccentric  position,  a  large  spherical  nucleus  which  was 
named  by  Purkinje  the  germinal  vesicle:  the  nuclear  con- 
tents exhibit  a  coarse  network  characteristic  of  the  phase 
of  complete  rest :  there  is  usually  but  a  single  nucleolus, 
which  is  quite  large  and  rounded,  and  was  called  by  Wag- 
ner the  germinal  spot. 

When  the  ovum  is  discharged  by  the  rupture  of  the  fol- 
licle u])on  the  surface  of  the  ovar^^  the  follicular  cavity  is 
at  first  filled  with  a  clot  of  blood.  It  is  quickly  invaded 
by  growths  from  the  wall  of  the  follicle  formed  in  part  of 
rapidlv  proliferating  cells  of  the  membrana  granulosa,  in 
part  of  folds  and  procesvses  from  the  theca :  it  has  been  as- 
serted that  the  interstitial  cells  previously  mentioned  also 
enter  to  a  large  extent  into  the  ingrowing  structure.  The 
result  is  the  formation  (about  the  shrunken  and  discolored 
clot  as  a  centre)  of  a  mass  ofmingled  cells  and  fibres  known 
as  a  corpus  luteum :  this  is  at  first  sharply  defined  by  the 
presence  of  the  theca,  but  gradual!}'  loses  its  definiteness, 
and  becomes  continuous  with  the  mass  of  the  ovary,  the 
peculiar  spurious  tissue  thus  formed  composing  quite  a 


238  PART    II.      HISTOLOGICAL    ANATOMY- 

large  part  of  that  organ  in  age.  The  corpus  luteum  formed 
concurrently  with  pregnancy  is  large  and  well-defined  and 
is  regarded  as  characteristic :  but  corpora  lutea  equally 
large  and  distinct  sometimes  (though  more  rarely)  occur 
under  other  conditions. 

The  oviducts,  commonly  termed  the  Fallopian  tubes, 

.  »> 
while  they  vary  in  form  in  the  several  regions  distinguish- 
ed by  the  anatomist,  are  quite  uniform  in  their  histologi- 
cal structure  throughout  their  whole  extent.  Continuous 
at  the  isthmus  with  the  uterus,  they  open  at  the  fimbriated 
extremities  into  the  peritoneal  cavit}^ :  they  consequently 
present  the  only  instance  of  direct  continuitx^between  a 
mucous  and  a  serous  surface.  Like  nearly  all  the  larger 
tubular  structures  in  the  body,  the  wall  is  divisible  into  a 
mucosa,  a  submucosa  and  a  musculosa;  to  which  is  added 
a  serosa  derived  from  their  investment  by  the  marginal 
fold  of  the  broad  ligament. 

The  mucosa  consists  of  a  well  developed  fibrous  mem- 
brane moderately  rich  in  elastic  fibres,  and  well  supplied 
with  blood  vessels  and  lymphatics,  which  supports  a  layer 
of  simple  columnar  ciliated  epithelium.  An  imperfectly 
developed  muscularis  mucosae,  consisting  of  longitudinal 
bundles  of  smooth  fibres,  is  also  present.  The  mucosa 
throughout  its  extent  is  thrown  into  longitudinal  folds 
which  in  the  ampulla  and  particularly  in  the  infundibulum 
are  very  extensive  and  have  secondary  folds  upon  their 
surfaces,  giving  to  the  cross  section  a  peculiar  arborescent 
appearance.  As  seen  in  such  sections  the  bases  of  these 
folds  often  present  the  appearance  of  tubular  glands ;  but 


CHAPTER  XIX.   F<i: PRODUCTIVE  ORGANS.      230 

true  srlands  are  not  present.  The  inner  surface  of  the  fun- 
briae  is  lined  with  the  mucosa,  while  the  outer  surface  is 
covered  with  the  serosa:  the  two  becomingconfluent  along 
the  sides. 

The  submucosa  is  a  simple  layer  of  areolar  tissue  of  but 
slight  depth.  It  is  continuous  with  that  of  the  uterus, 
like  which  it  contains  small  ganglia  and  scattered  multi- 
polar cells,  forming  the  rudiments  of  a  plexus.  The  mus- 
culosa  consists  of  an  inner  circular  and  an  outer  longitu- 
dinal layer  of  smooth  muscular  fibres,  the  latter  being  but 
imperfectly  developed.  The  serosa  consists  of  a  thin  fib- 
rous membrane  supporting  the  serous  endothelium  charac- 
teristic of  the  surface  of  the  peritoneum. 

Attached  to  the  extremity  of  the  tube  or  to  one  of  the 
fimbriae  is  frequently  found  a  pedunculated  cyst  or 
stalked  hydatid  of  Margagni.  It  is  the  homologue  of  the 
body  bearing  the  same  name  in  the  male  reproductive  ap- 
paratus and  resembles  it  in  general  structure,  the  cavity 
of  the  sac  and  also  of  the  pervious  portion  of  the  stalk 
being  lined  with  cuboidal  or  columnar  epithelium. 

Situated  in  the  broad  ligament  between  the  ovar\'  and 
the  ampulla  of  the  oviduct  is  a  well-defined  mass  of  ir- 
regularly convoluted  tubules,  known  as  the  parovarium. 
It  is  also  called,  from  its  discoverer,  the  organ  of  Rosen- 
mueller:  Waldeyer  has  proposed  for  it  the  name  of  the 
epoophoron.  The  constituent  tubules  are  lined  with  low 
columnar  epithelium :  they  converge  toward  each  other, 
without  uniting,  at  the  ends  nearest  the  ovary :  the  other 


240  PART  II.      HISTOLOGICAL   ANATOMY. 

extremities  diverge  somewhat,  and  terminate  in  a  longi- 
■  tudinal  tube  which  runs  parallel  with  the  oviduct:  in  some 
of  the  lower  mammals  this  tube  is  quite  extensive  and  is 
known  as  the  duct  of  Gartner,  a  term  also  applied  to  it 
in  the  human  subject  by  many.  Somewhat  nearer  to  the 
uterus  than  the  parovarium  a  smaller  and  more  irregular 
group  of  rudimentary  tubules  occurs,  similar  in  structure 
to  those  just  described.  These  have  been  designated  by 
Waldeyer  the  paroophoron.  The  homologies  of  these  rudi- 
ments will  be  discussed  later. 

The  uterus  presents  but  two  distinct  regions  histologi- 
calh%thefundusandbody  agreeing  in  structure  and  differ- 
ing from  the  cervix.  The  most  characteristic  features  of 
the  upper  region  are  found  in  the  mucosa,  the  stroma  of 
which  is  greatly  modified,  while  the  muscularis  attains  a 
greater  development  than  does  the  structure  bearing  that 
name  in  any^other  portion  of  the  body.  The  surface  is  inves- 
ted with  a  single  layer  of  columnar  ciliated  cells  directly  con- 
tinuous with  that  lining  the  Fallopian  tube.  Beneath  this 
is  a  very  thick  mucous  membrane  containing  but  a  very 
small  quantity  of  fibres  and  composed  in  large  part  of 
spindle  shaped  cells  similar  to  those  found  in  the  stroma  of 
the  ovary,  loosely  interwoven:  the  spongy  mass  so  formed 
contains  numerous  lymph  spaces  and  leucocytes.  Im- 
bedded in  it  are  great  numbers  of  tubular  uterine  glands, 
wavy  or  convoluted  in  their  course,  not  infrequently'' 
branched,  and  penetrating  to  the  base  of  the  stroma  and 
quite  frequently  between  the  bundles  of  fibres  of  the  mus- 
cular layer:  they  are  bounded  by  a  delicate  basement  mem- 


CHAPTER    XIX.      REPRODUCTIVE   ORGANS.  24-1 

brane  which  supports  columnar  cells  similar  to  those  lin- 
ing the  uterine  wall :  near  the  blind  extremity  of  the  tube 
the  columnar  cells  entirely  till  its  cavity:  but  throughout 
the  greater  part  of  its  extent  there  is  a  distinct  lumen. 

The  rnuscularis  rnucosae  is  the  chief  muscular  coat  of  the 
uterine  wall;  greatly  developed  at  all  times,  it  is  enor- 
moush'  hypertrophied  during  pregnancy,  parth'  by  the 
great  increase  in  number.  It  consists  of  bundles  of  fibres 
interwoven  with  a  sparing  amount  of  interstitial  connect- 
ive tissue,  and  running  in  various  directions:  their  dispo- 
sition is  apparently  quite  irregular,  and  cannot  be  de- 
scribed briefly  with  clearness :  it  is,  moreover,  subject  to 
considerable  variations:  it  can  perhaps  be  best  understood 
bv  regarding  it  as  consisting  chiefly  ot  circularly  disposed 
bundles  which  are  arranged  on  the  fundus  in  two  sets,  one 
concentric  to  the  insertion  of  each  of  the  two  oviducts, 
and  which  become  gradually  combined  to  form  a  single 
set  as  the}'  approach  the  lower  extremity  of  the  body. 

There  is  associated  with  the  great  development  of  the 
rnuscularis  mucosae  a  corresponding  reduction  of  the  sub- 
mucosa,  there  being  less  independent  movement  of  the 
mucosa  and  musculosa  in  this  case  than  in  almost  any  of 
the  similar  hollow  structures.  There  is,  however,  a  dis- 
tinct zone  of  connective  tissue  discernible  just  exterior  to 
the  muscularis  mucosae  characterized  particularly  by  the 
presence  of  numerous  bloodvessels  and  lymphatics,  and 
by  scattered  nervous  elements.  It  is  of  such  slight  extent 
as  to  be  by  some  described  as  wanting. 

The  musculosa  consists  of  two  distinct  laj^ers  of  smooth 
fibres  both  quite  thin  and  varying  in  their  relative  devel- 


242  PART  II.      HISTOLOGICAL  ANATOMY, 

opment  in  different  parts  of  the  organ.  The  inner  or  cir- 
cular layer  is  the  more  uniform  in  thickness  and  in  the  ar- 
rangement of  its  fibres :  the  fibres  of  the  outer  laver  are  in 
the  main  longitudinally  disposed,  but  are  somewhat  ir- 
regularly arranged  upon  the  fundus,  in  accordance  with 
its  irregularities  of  form.  This  laj-er  also  gives  off  bundles 
of  muscular  fibres  extending  out  into  the  ligaments  of  the 
uterus.  The  musculosa  is  invested  by  a  serosa  which  is  a 
continuation  of  the  peritoneum. 

The  cervix  differs  from  the  region  just  described  chieflv 
in  the  structure  of  the  mucosa,  the  stroma  of  which  is 
much  firmer  and  richer  in  fibres,  both  white  and  elastic, 
the  membrane  being  thrown  into  characteristic  folds,  and 
in  the  lower  portion  beset  wnth  minute  papillae.  The  up- 
per two-thirds  is  lined  with  columnar  ciliated  epithelium 
continuous  with  that  of  the  upper  regions :  this  passes 
below  into  the  stratified  squamous  epithelium  which  in- 
vests the  papillated  lower  third.  There  are  present  both 
tubular  and  saccular  glands  said  to  be  lined  in  each  case 
with  columnar  ciliated  epithelium :  the  saccular  glands 
contain  also  goblet  cells  and  secrete  the  thick  mucus  found 
in  the  cervix.  Here  and  there  spheroidal  bodies  filled  with 
a  clear  yellowish  fluid  can  be  seen  with  the  naked  eye: 
they  are  probably  occluded  and  enlarged  mucous  glands, 
and  are  known  as  ovula  Nabothi.  The  muscularis  mu- 
cosae is  well  developed  and  consists  chiefly  of  circular 
bundles :  these  are  accumulated  in  greater  numbers  at  t2 
upper  and  lower  extremities  of  the  cervix  to  form  the 
sphincters  of  the  regions  in  question. 

The  submucosa  is  not  conspicuous.    The  musculosa  re- 


'1/ 


tM/< 


CHAPTER    XIX.      REPRODUCTIVE   ORGANS.  24-3 

sembles  in  structure  the  same  portion  of  the  wall  of  the 
body  of  the  uterus:  the  imier  circular  and  outer  Ion  git  u- 
dinal  layers  are  clearly  defined.  The  region  of  the  cervix 
toward  the  rectum  is  invested  with  a  peritoneal  serosa: 
that  toward  the  bladder  is  separated  from  that  organ  b\' 
an  adventitia  of  areolar  tissue.  The  portion  of  the  cer- 
vix that  projects  into  the  vagina  to  form  the  OS  uteri 
agrees  in  structure  on  its  inner  aspect  with  the  cervix  :  on 
its  outer  it  is  a  continuation  of  the  vaginal  wall. 

The  vagina  differs  from  the  uterus  in  structure  in  accor- 
dance with  its  differing  and  double  function,  it  serving  at 
once  as  the  channel  by  which  the  male  reproductive  ele- 
ments are  brought  into  proximity  with  the  female  ele- 
ments, and  as  the  avenue  of  discharge  for  the  foetus  at  its 
maturity.  Its  wall  is  muscular,  dilatable,  and  highly 
elastic,  somewhat  erectile,  and  provided  with  a  definite 
reinforcement  of  adenoid  tissue ;  a  feature  possessed  by  no 
other  portion  of  the  reproductive  tract. 

The  mucosa  of  the  vagina  is  lined  with  stratified  squa- 
mous epithelium,  which  rests  upon  a  thick  mucous  mem- 
brane. The  surface  of  the  latter  is  beset  with  minute  pa- 
pillae which  project  into  the  deeper  portions  of  the  epithe- 
lium but  do  not  produce  a  noticeable  roughness  of  its  sur- 
face, the  only  irregularities  observable  being  those  due  to 
the  well-marked  folds  or  rugae.  The  upper  portion  is  a 
dense  fibrous  layer  rich  in  elastic  fibres :  below  this  in  the 
rugae  are  networks  of  large  veins  supported  by  fibrous 
tissue  containing  numerous  bundles  of  smooth  muscular 
fibres  which  may  be  regarded  as  representing  the  muscu- 


244  PART  II.      HISTOLOGICAL  ANATOMY. 

laris  mucosae,  elsewhere  absent :  a  ridge  of  rudimentary 
erectile  tissue  is  thus  formed  beneath  each  ruga.  Leuco- 
cytes abound  in  the  mucosa,  and  scattered  nodules  of  ade- 
noid tissue  are  found  :  in  the  anterior  wall  near  the  orifice 
there  is  a  \vell-defined  adenoid  layer.  Special  nerve  termi- 
nals, the  genital  corpuscles  of  Krause,  as  found  in  the 
mucosa  of  the  vagina.  It  is  doubtful  whether  glands  of 
any  sort  are  present. 

The  submucosa  is  quite  loose  in  structure  and  contains 
a  venous  network  whose  meshes  run  chiefly  in  the  direc- 
tion of  the  vagina.  Beyond  the  submucosa  is  the  mus- 
culosa,  which  is  not  sharply  defined,  as  in  most  cases,  in- 
to distinct  strata :  the  inner  bundles  are  in  the  main  circu- 
larly disposed,  and  the  outer  bundles  longitudinally;  the 
two  regions  being,  however,  blended  by  numerous  oblique 
bundles.  A  well-marked  fibrosa  invests  the  musculosa :  it 
is  composed  largely  of  elastic  tissue  and  is  best  developed 
on  the  anterior  wall :  it  also  contains  an  extensive  plexus 
of  large  veins  intermingled  with  bundles  of  smooth  mus- 
cular fibres  and  forming  a  layer  of  erectile  tissue  best  de- 
veloped near  the  lower  extremity. 

I^he  hymen  agrees  in  its  structure  with  a  fold  of  the 
mucosa  of  the  vagina,  and  can  perhaps  be  regarded  as  de- 
rived therefrom,  though  its  presence  in  rare  instances  in 
cases  of  absence  of  the  vagina  has  caused  this  mode  of 
origin  to' be  questioned  by  those  who  regard  it  as  a  fold  of 
the  skin  of  the  vestibule. 

The  vulva  includes  a  number  of  parts  or  regions  each 
characterized  by  certain  histological  features  worthy  of 


CHAPTER    XIX.      REPRODrCTIYE   ORGAN'S.  24." 

brief  mention.  The  surface  of  the  area  known  as  the  ves- 
tibule is  covered  by  a  mucous  membrane  continuous  with 
that  of  the  vagina  and  of  the  urethra  at  their  respective 
orifices.  It  is  covered  with  stratified  squamous  epitheli- 
um, contains  numerous  elastic  fibres,  and  is  feebly  erectile: 
it  also  contains  numerous  simple  mucous  glands.  At  the 
lower  limit  of  the  vestibular  area  are  seen  on  either  side  of 
the  vaginal  orifice  the  openings  of  the  ducts  of  the  glands 
of  Bartholin,  small  racemose  glands  of  the  mucous  type 
homologous  with  the  glands  of  Co  wper  in  the  male  sub- 
ject. Beneath  the  mucous  membrane  of  the  vestibule  and 
somewhat  external  to  the  proper  limits  of  the  vestibular 
surface  are  paired  elongated  masses  of  erectile  tissue,  the 
bulbi  vestibuli,  whose  converging  upper  extremities  are 
continuous  with  smaller  plexuses  whose  vessels  are  conflu- 
ent above  with  those  of  the  glans  clitoridis.  The  bulbar 
regions  may  be  regarded  as  corresponding  to  the  bilateral 
bulbous  portions  of  the  corpus  spongiosum  of  the  male 
subject. 

The  clitoris  consists  of  two  small  corpora  cavernosa 
identical  in  structure  and  relations  wnth  those  of  the  male, 
and  a  small  glans  of  spongy  erectile  tissue,  which  is  of 
course  imperforate.  Its  surface  contains  numerous  geni- 
tal corpuscles.  Its  preputial  fold  is  continuous  with  the 
upper  and  its  fraenum  with  the  lower  of  the  anterior  di- 
visions of  the  labia  minora.  The  latter,  while  resembling 
the  surface  of  the  vestibule  in  color  and  texture,  may  be 
regarded  as  folds  of  the  skin:  the}' contain  numerous  large 
sebaceous  glands,  but  sweat  glands  are  wanting,  as  are 
also  hairs :  their  inner  surface  contains  numerous  genital 


246  PART  II.      HISTOLOGICAL  ANATOMY, 

corpuscles.  Fat  is  wanting  in  the  subcutaneous  connec- 
tive tissue,  but  large  irregular  venous  channels  are  present, 
with  smooth  muscular  fibres,  composing  here  as  elsewhere 
a  loose  form  of  erectile  tissue.  The  labia  majora  are  well 
defined  folds  of  the  skin  whose  inner  surfaces  resemble  in 
appearance  the  outer  surfaces  of  the  labia  minora,  with 
which  they  are  confluent,  but  differ  from  them  in  the  pre- 
sence of  occasional  modified  sweat  glands  and  minute 
hairs,  associated  with  the  sebaceous  glands  common  to 
both.  The  thick  and  rounded  margin  of  the  fold  affords 
a  gradual  transition  from  the  moist  stratified'squamous 
epithelium  of  the  mucous  type  found  upon  the  greater  por- 
tion of  the  vulvar  surface  to  the  epidermis  of  the  skin, 
with  which  its  outer  surface  agrees  in  general  struct- 
ure. In  the  deeper  portions  of  the  integument  of  the  labia 
majora  is  found  a  layer  of  tissue  similar  to  that  forming 
the  dartos  tunic  of  the  male  scrotum,  with  which  the  labia 
correspond.  Like  the  divisions  of  that  structure  they 
converge  above  to  be  united  upon  the  pubic  eminence  in 
the  region  known  as  the  mons  veneris,  characterized,  as 
in  the  male,  by  an  abundance  of  coarse  curling  hairs,  of 
numerous  enlarged  sudoriparous  glands,  and  by  a  dense 
mass  of  subcutaneous  fat. 


The  mammary  glands,  while  essentially  tegumentary 
organs,  differ  so  greatly  from  all  other  dermal  glands  as 
to  require  consideration  apart  from  the  latter:  their  func- 
tional relations  to  reproduction  render  it  appropriate  to 


CIIAI'TEK    XIX.      KlirKODlCTIVH    ORGANS.  24-7 

discuss  them  at  this  time.  I'sualh'  but  not  always  rudi- 
mentary in  the  malesuljjcct,  they  are  normally  but  not  in- 
variably fully  developed  in  the  female.  Each  mammary 
gland  so  called  is  in  reality  an  aggregate  of  fifteen  or  tvventy 
distinct  glands,  if  we  regard  that  term  as  strictly  desig- 
nating a  secretory  body  provided  with  a  duct  which  opens 
independently,  since  each  of  the  ducts  opens  by  a  separate 
orifice  upon  the  skin  of  the  nipple:  but  their  union  into  a 
single  anatomical  structure  is  so  intimate  that  it  is  more 
convenient  to  designate  each  of  these  regions  as  a  lobe  ot 
a  compound  racemose  gland.  The  structure  of  these  lobes 
and  of  the  whole  organ  varies  greatly  in  relation  to  function- 
al activity ;  but  it  is  characteristic  of  it  at  all  times  that  it 
possesses  an  unusuallv  large  proportion  of  connective  tis- 
sue and  fat  in  its  composition. 

Before  the  gland  has  been  called  into  functional  activity 
the  lactiferous  ducts  are  present,  as  well  as  the  ducts  ot 
the  lobules  by  whose  confluence  they  are  formed  :  the  lat- 
ter have  at  the  extremities  rudimentary  acini,  which  are, 
however,  relatively  few  in  number,  and  consist  of  masses 
of  epithelial  cells.  Acini,  lobules,  and  lobes  are  alike  im- 
bedded in  an  extensive  stroma  of  connective  tissue  which 
forms  stout  septa  not  only  between  the  lobes  but  between 
their  subdivisions  as  well ;  while  a  considerable  amount  of 
subcutaneous  and  interstitial  adipose  tissue  is  present. 

As  pregnancy  advances  the  acini  become  larger  and 
more  numerous,  still  consisting,  however,  of  solid  masses 
of  cells.  At  the  time  of  delivery  the  central  cells  undergo 
fatt}'  degeneration  and  form  the  colostrum  corpuscles  of 
the  milk  of  commencing  lactation.     In   the  fully  active 


248  PART  II.      HISTOLOGICAL  ANATOMY. 

gland  the  acini  are  sgherpidal,  comparatirely  large,  anJ 
are  lined  b}^  a  single  layer  of  cells  which  when  at  rest  are 
flattened,  but  during  secretion  become  cuboidal  or  colum- 
nar in  form,  their  extremities  containing  one  or  more  large 
oil-droplets  in  each  instance:  the  latter  are  liberated  by  the 
rupture  of  the  cell  substance,  a  portion  of  w^hich  is  prob- 
abh^  contributed  to  the  secretion.  The  basement  mem- 
brane upon  which  the  epithelium  rests  consists  of  an  en- 
dothelioid  layer  of  connective  tissue  corpuscles :  the  intra- 
lobular stroma  is  greatW  reduced  in  proportional  quan- 
tity. The  terminal  branches  of  the  ducts  have  a  thin  base- 
ment membrane  like  that  of  the  alveoli,  which  is  lined  by 
a  single  layer  of  flattened  cells,  their  appearance  resemb- 
ling that  of  the  ductules  of  the  salivary  glands :  the  larger 
lactiferous  ducts  have  stouter  walls,  and  cuboidal  epithe- 
lium ;  these  empty  into  the  still  larger  channels,  the  galac- 
tophorous  ducts,  one  of  w^hich  leads,  as  has  been  stated, 
from  each  lobe  to  an  independent  orifice  upon  the  skin  of 
the  nipple.  These  large  ducts  have  stout  walls  of  fibrous 
and  elastic  tissue,  containing  a  few  smooth  muscular 
fibres :  they  are  lined  with  columnar  epithelium  save  in 
their  outermost  portions,  where  the  epithelium  becomes 
stratified.  Each  has  an  enlargement,  the  ampulla,  near 
its  termination  at  the  nipple,  whose  structure  does  not 
differ  from  that  of  the  rest  of  the  duct. 

In  the  intervals  between  lactation  the  mammary  glands 
assume  a  resting  condition  in  many  respects  similar  to 
their  primitive  state.  They  always  contain,  however,  a 
smaller  amount  of  dense  fibrous  tissue  and  a  larger 
amount  of  fat,  and  are  consequently  much  less  firm  in 


CHAPTER    XIX.      RKPRODUCTIVE  ORGANS.  240 

texture.  At  the  close  ot  the  reproductive  period  they  be- 
gin to  undergo  a  retrograde  metamorphosis,  the  acini  and 
smaller  ducts  disappearing,  the  larger  ducts  collapsing, 
and  the  shrunken  organ  consisting  chiefly  of  a  mass  of 
connective  tissue  and  fat. 

The  nipple  is  a  cvlindrical  projection  from  the  surface 
of  the  gland,  covered  with  deeply  pigmented  skin  and 
composed  of  the  extremities  of  the  lactiferous  ducts,  as- 
sociated bloodvessels,  and  smooth  muscular  fibres  ar- 
ranged in  circular  and  longitudinal  bundles.  The  der- 
mal papillae  are  rich  in  nerve  terminals:  there  is  no 
subcutaneous  fat :  scattered  in  the  surface  are  the  small 
racemose  glands  of  Montgomery :  the  areola  at  its  base 
contains  sweat-glands  and  numerous  sebaceous    glands. 


The  intimate  relations  existing  between  the  urinary 
and  reproductive  systems  and  the  homologies  between 
the  male  and  female  sexual  organs  cannot  be  clearly 
stated  without  a  description  of  their  formation  in  the 
embr^'o,  accompanied  by  a  statement  of  some  at  least 
of  the  facts  of  their  comparative  anatoni}'.  No  attempt 
will  be  made  at  this  time  to  discuss  systematically 
either  the  embr\'ologv'  or  the  morphology'  of  these  or- 
gans, but  such  account  of  each  will  be  alone  given  as 
appears  necessar\'  to  the  intelligent  comprehension  of 
the  relations  and  homologies  above  referred  to. 

The  structure  fundamental  to  the  whole  of  the  uro- 
genital apparatus  is  what  is  known  as    a    nephridium. 


250  PART   II.      HISTOLOGICAL   ANATOMY. 

In  the  great  majority  of  the  classes  of  the  higher  inver- 
tebrates the  elimination  of  nitrogenous  waste  products 
takes  place  through  the  agency  of  organs  designated 
by  that  term.  A  nephridium  is  essentially  a  tubular 
structure  opening  at  one  extremity  upon  the  surface  of 
the  bod}'  (either  directly  or  indirectly),  and  at  the  other 
communicating  with  the  bod\^  cavity,  or  coelom,  by  a 
more  or  less  funnel-shaped  extremity  termed  the  ne- 
phrostome.  Its  wall  is  lined  with  an  epithelium  which 
is  glandular  throughout  a  large  portion  of  its  extent : 
the  funnel-shaped  internal  opening  is  commonly  and  the 
larger  portion  of  the  rest  of  the  tube  frequently  cili- 
ated, the  ciliary  movement  invariably  sweeping  toward 
the  external  opening.  The  tube  may  be  simple  and 
quite  direct  in  its  course,  or  long,  convoluted,  and  di- 
vided into  specialized  regions.  Where  a  well  developed 
vascular  system  is  present  the  more  or  less  coiled  ne- 
phridial  tube  is  usualh^  provided  with  a  rich  net-work 
of  capillaries.  Nephridia  may  be  simple:  a  single  pair 
opening  right  and  left  on  the  surface  of  the  body,  as  in 
many  mollusks;  or  a  pair  being  found  in  each  of  the 
majority  of  the  segments  of  the  body,  as  in  the  annelids, 
where  they  were  first  observed,  and  designated  from 
their  disposition  segmental  organs:  or  they  ma}'  be 
compound,  a  number  of  funnel-bearing  and  ciliated  tu- 
bules opening  symmetrically  into  lateral  tubes  which 
discharge  [the  secretion  of  the  tubules  upon  the  surface 
of  the  body  or  into  a  posterior  cloacal  sac,  as  in  the  roti- 
fers. Without  entering  into  a  discussion  of  the  homolo- 
gies that  may  exist  between  the  nephridia  of  the  various 


CHAPTER    XIX.    REPRODUCTIVE   ORGANS  251 

classes  of  invertebrates,  or  between  those  of  either  of 
them  and  those  of  the  vertebrates,  it  may  be  said  that  the 
urinary  apparatus  of  the  hitter  conforms,  in  a  general 
way  at  least,  to  the  plan  last  described. 

The  gonads,  in  which  the  reproductive  elements  are 
formed,  are  in  most  classes  of  Metazoa  developed  within 
the  body-cavity,  or  coelom,  into  which  thev  ])rotrude, 
and  into  which  the  reproductive  elements,  when  fully 
mature,  are  in  many  species  discharged.  In  some  of  the 
lower  forms  where  this  takes  place,  and  in  which  the  re- 
productive elements  are  produced  in  immense  numbers, 
the  body  wall  becomes  distended  by  their  presence  and 
finalU'  liberates  them  by  rupturing,  the  parent  organism 
being  thereby  destroyed ;  a  condition  comparable  to  that 
of  the  so-called  annual  plants,  which  grow,  blossom,  ripen  a 
single  crop  of  seeds,  and  die.  In  other  cases  the  ne- 
phridia  serve  as  channels  for  the  escape  of  the  reproduc- 
tive elements :  it  will  be  noted  that  the  small  and  actively 
moving  spermatozoa  would  pass  far  more  readih' 
through  the  nephridial  tubules  than  the  large  and  passive 
ova;  and  it  is  probabK"  true  in  the  case  of  some  species 
that  the  female  is  destroyed  by  the  rupture  of  the  body 
wall  after  the  ripening  of  the  first  crop  of  ova,  while  the 
male  elements  are  liberated  through  the  nephridia,  the  pa- 
rent organism  surviving.  In  most  of  the  higher  classes  of 
animals  there  are  distinct  channels  of  discharge,  the  gon- 
aducts,  for  the  reproductive  elements :  these  are  in  some 
cases  clearly  formed  by  the  modification  of  nephridia. 

The  vertebrate  urinary  aparatus  consists  in  effect  of  a 
series  of  nephridial  tubules  arranged  along  the  dorsal  wall 


252  PART   n.    HISTOLOGICAL   ANATOMY. 

of  the  body  cavity  on  either  side  of  the  mid-plane  and 
opening  into  right  and  left  ducts  of  discharge.  The  dis- 
position, form  and  relations  of  the  tubules,  and  the  origin 
and  position  of  the  duct  of  discharge  of  the  functional  kid- 
ney undergo  important  modifications  in  the  various 
classes  of  vertebrates.  Taking  the  group  as  a  whole  the 
series  of  tubules  may  be  said  to  be  divided  into  three  reg- 
ions, the  pronephros  or  head-kidney,  the  mesonephros 
or  primitive  kidney,  and  the  metanephros  or  permanent 
kidney  of  the  higher  vertebrates.  Each  of  these  regions 
needs  brief  consideration. 

The  pronephros  is  best  developed  in  the  anamnia  (cyclos- 
tomes,  fish-like  vertebrates,  and  amphibia),  in  all  of  which 
it  is  present  as  a  well-defined  structure  in  the  embr3'o,  and 
in  some  of  which  it  is  functional  in  the  adult.  The  tubules 
are  always  few  in  number,  sometimes  but  one  on  each 
side,  open  upon  the  body  cavity  by  distinct  nephrostomes, 
and  discharge  into  ducts  leading  to  the  cloaca  and  known 
as  the  segmental  ducts.  Near  the  opening  of  the  nephro- 
stomes there  is  found  on  each  side  a  process  of  the  coelo- 
mic  wall  containing  a  mass  of  capillaries  and  constituting 
a  prominent  glomus  :  the  region  of  the  coelom  where  this 
occurs  is  either  partialK'  or  in  some  cases  wholly  con- 
stricted off  from  the  rest  of  the  body-cavit}-  so  as  to  form 
"what  is  practically  an  enormous  Malpighian  bod}':" 
thus  foreshadowing  the  arrangement  found  in  the  perma- 
nent kidncA'  of  the  mammals.  The  development  of  the 
pronephros  is  embr\'onic  and  rudimentarv  in  the  sharks 
and  their  allies  among  the  fish-like  vertebrates,  and  in  all 
amniota  (reptiles,  birds,  and  mammals):  in  the  last  named 


CHATTER    XIX    REPRODUCTIVE   ORGANS.  253 

class  it  is  doubtfully  represented  in  the  adult  male  and  fe- 
male by  rudimentary  structures  connected  with  the 
surprarenal  capsule. 

The  mesonephros  is  the  functional  kidnc}-  of  all  anam- 
nia  except  the  sharks  and  their  allies.  In  this  class  and  in 
the  various  classes  of  the  amniota  it  is  represented  by  the 
embryonic  structure  termed  the  Wolffian  body.  It  con- 
sists primarily  of  serially  arranged  tubules  plainh'  homol- 
ogous with  those  of  the  pronephros,  although  their  mode 
of  development  is  not  identical,  being  somewhat  acceler- 
ated ;  a  point  of  importance  in  connection  with  the  origin 
of  the  metanephros.  These  tubules  become  variously 
coiled  and  convoluted,  and  may  give  rise  to  branches  like 
themselves:  in  many  anamnia  they  open  upon  the  peri- 
toneal surface  by  nephrostomes :  the\'  have  associated 
with  them  glomeruli  resembling  in  their  origin  the  glom- 
us just  described  in  connection  with  the  pronephros :  in 
some  cases  several  tubules  open  upon  a  single  glomerulus. 
The}'^  discharge  into  a  modification  of  the  segmental  duct 
known  as  the  mesonephric  or  Wolffian  duct:  it  differs 
from  the  primary  segmental  duct  in  undergoing  (in  some 
species)  longitudinal  cleavage  to  give  rise  to  a  second 
canal  called  the  Muellerian  duct. 

The  ripened  gonads  of  many  anamnia  discharge  the  re- 
productive elements  into  the  body  cavity  by  rupture, 
from  which  the\'  escape  through  openings  posteriorly  and 
ventrally  placed  and  known  as  abdominal  pores.  In 
other  cases  abdominal  pores  are  wanting:  in  the  female 
the  ova  pass  out  through  the  Mullerian  duct  or  oviduct ; 
in  the  male  the  testis  lies  opposite  the  anterior  end  of  the 


254  PART  II.   HISTOLOGICAL  ANATOMY. 

primitive  kidney,  or  Wolffian  bod}' ;  diverticula  grow  out 
from  the  Wolffian  tubules  of  that  region  and  become  con- 
nected with  the  testis,  forming  channels  of  escape  for  the 
spermatozoa,  which  thus  reach  the  Wolffian  duct ;  the 
latter  therefore  functions  both  as  a  ureter  and  a  spermi- 
duct. 

In  the  selachians  (the  sharks  and  their  allies),  and  in  all 
amniota  a  diverticulum  is  given  offfrom  the  posterior  por- 
tion of  the  Wolffian  duct.  In  the  former  class  this  be- 
comes connected  with  a  mass  of  serially  disposed  tubules 
arising  posterior  to  the  Wolffian  body,  the  metane- 
phros,  or  permanent  kidney ;  the  duct  in  question  being 
the  true  ureter.  In  the  amniota  the  ureter  grows  out  to- 
ward a  mass  of  cells  in  which  true  uriniferous  tubules 
with  their  associated  Malpighian  corpuscles  are  being 
formed.  It  is  not  certain  that  this  structure  is  strictly 
homologous  with  the  selachian  metanephros:  it  may,  per- 
haps, be  so  regarded,  the  difference  in  the  mode  of  devel- 
opment being  due  to  an  intensified  acceleration  of  growth 
similar  to  that  already  mentioned  in  the  case  of  the  Wolff- 
ian body.  The  latter  structure  never  acts  as  a  kidney  in 
the  higher  vertebrates,  its  sole  function  being  connected 
with  the  transmission  of  the  reproductive  elements  of  the 
male :  the  Wolffian  duct  becomes  the  spermiduct  or  vas 
deferens :  in  the  female  the  whole  structure  is  functionless 
and  rudimentary. 

This  digression  into  the  field  of  comparative  anatomy 
W'ill  not  be  without  value  if  it  aids  in  making  clear  the 
brief  statement  of  the  development  and  homologies  of  the 
male  and  female  urogenital  apparatus  of  the  human  sub- 


CHAPrEK    XIX     KKPRODUCTIVE   ORGANS.  255 

ject    based    upon    the    facts  nicntloncd    therein,  whicli   is 
now  to  be  entered  upon. 

Early  in  the  development  of  the  embryo  there  may  be 
seen  ji^rowing  downward  from  the  dorsal  wall  of  the  body 
cavity  on  eit]ier  side  a  well-defined  eminence,  the  Wolffian 
orexcretory  ridge:  alongitsbasearefound  two  ducts,  the 
Wolffian  and  Mitllcrian.  Just  niesad  of  the  excretor\' ridge, 
and  separated  from  it  by  so  shallow  a  groove  at  first  as 
almost  to  seem  a  modification  of  its  mesial  surface,  is  a 
second  and  smaller  ridge,  the  germinal  ridge.  The  latter 
becomes  the  gonad  :  the  epithelium  of  its  surface  early  be- 
comes columnar  and  shows  here  and  there  numerous  prim- 
ordial ova;  the  characteristiccellsbcingso termed  without 
regard  to  the  future  sex.  The  changes  of  position  which 
the  gonads  undergo  in  each  sex  will  be  mentioned  later. 

The  embryonic  pronephros  is  formed  at  the  anterior  ex- 
tremity of  the  excretory  ridge.  The  posterior  portion,  as 
it  developes  backward,  becomes  the  blastema  or  mass  of 
cells  in  which  the  uriniferous  tubules  of  the  kidney  are 
formed  :  it  is  joined  by  the  ureter,  whose  extremit}' branches 
and  forms  the  pelvis  and  calyx  of  the  kidne\',  and  possibly 
to  some  extent  the  collecting  tubules:  the  exact  relation 
of  the  two  intergrowing  structures  is  not  j-et  fulh' known. 
At  first  the  most  posterior,  the  kidne}^  gradual!}^  advances 
in  position  until  it  occupies  a  location  anterior  to  all  the 
associated  structures. 

The  middle  region  of  the  excretory  ridge  becomes  the 
Wolffian  bod\' :  in  both  sexes  it  develops  as  a  series  of 
transverse  tubules  in  relation  with  the  longitudinal  Wolff- 
ian duct.    In  the  male  the  tubules  of  the  anterior  portion 


256  PART  II.   HISTOLOGICAL  ANATOMY. 

become  connected  with  the  testis,  forming  the  coni  vascu- 
losi  of  the  epididymis,  and  possibly  the  rete  testis  as  welh 
those  of  the  posterior  portion,  corresponding  to  the  strictly 
renal  portion  of  the  primitive  kidney  of  the  anamnia,  be- 
come rudimentary  and  form  the  vasa  aberrantia  and  the 
paradidymis,  or  organ  of  Giraldes.  The  Wolffian  duct  be- 
comes the  convoluted  tubule  of  the  epididymis  with  its 
continuation,  the  vas  deferens,  the  latter  giving  off  the  semi- 
nal vesicles  as  it  is  transformed  into  the  ejaculatory  duct. 
In  the  female  the  whole  structure,  being  functionless  be- 
comes rudimentary :  the  anterior  portion  of  the  tubules, 
corresponding  to  the  epididymis,  becomes  the  parovarium 
or  epoophoron ;  the  posterior  portion  becomes  the  paro- 
ophoron :  the  Wolffian  duct  becomes  the  rudimentary  duct 
of  Gartner. 

The  Miillerian  duct  undergoes  corresponding  differentia- 
tion. In  the  female,  where  it  is  functional,  the  upper  por- 
tion terminates  in  the  fimbriated  extremity:  the  middle 
portion  becomes  the  Fallopian  tube,  or  oviduct  in  the 
strict  sense.  The  lower  portions  coalesce  on  the  mid-line, 
the  upper  region  of  the  median  structure  forming  the 
uterus  and  the  lower  the  vagina:  a  discussion  of  the  mode 
of  their  coalescence  would  take  us  again  into  the  domain 
of  comparative  anatomy.  In  the  male  the  Miillerian  duct 
is,  like  the  Wolffian  duct  of  the  female,  altogether  function- 
less.  It  early  disappears  throughout  the  greater  part  of 
its  extent:  the  upper  portion  is  perhaps  represented  by  the 
sessile  hydatid  of  the  testis :  the  coalesced  lower  portions 
form  the  uterus  masculinus,  which  sometimes  showstraces 
of  a  vaginal  region. 


CHAPTER    XIX.      REPRODUCTIVK   ORCANS.  2o7 

In  all  amniota  there  is  found  very  early  in  embryonic 
life  a  vesicular  diverticulum  of  the  ventral  wall  of  the  in- 
testine at  a  point  near  its  posterior  termination:  this  out- 
growth is  called  the  allantois.  In  the  mammals  a  consid- 
erable portion  of  it  is  enclosed  within  the  body  cavity:  the 
distal  portion  of  this  becomes  enlarged  to  form  the  urin- 
ary bladder;  a  narrow  region  corresponding  to  the  true 
urethra  of  both  sexes  connects  it  with  a  proximal  enlarge- 
ment termed  the  urogenital  sinus:  the  latter  opens  in 
common  with  the  intestine  into  a  short  cloaca  formed  by 
a  depression  of  the  ventral  surface  of  the  body.  The  Miil- 
lerian  and  Wolffian  ducts  open  into  the  sinus:  the  ureters 
at  first  open  into  the  Wolffian  duct,  from  which  they  are 
derived ;  later  they  open  independently  into  the  sinus :  as 
growth  advances  they  shift  their  position  upward,  until 
they  reach  their  permanent  point  of  discharge  on  the  sur- 
face of  the  bladder. 

The  mammalian  cloaca  is  from  the  first  a  very  shallow 
depression  :  a  transverse  fold  soon  separates  it  into  a  pos- 
terior anal  portion,  and  an  anterior  region  which  is  the 
continuation  ofthe  urogenital  sinus.  Just  in  front  of  it  there 
is  found  the  genital  eminence,  whose  posterior  surface  is 
grooved,  the  margins  of  the  groove  forming  the  genital 
folds;  in  front  of  its  base  the  skin  of  the  pubis  is  thrown 
into  a  thick  fold  (the  future  mons  veneris)  which  is  pro- 
longed backward  right  and  left  in  the  genital  ridges.  In 
the  male  the  genital  eminence  elongates ;  corpora  caver- 
nosa are  formed  in  it :  the  genital  folds  coalesce  from  be- 
hind foward,  converting  the  groove  on  the  posterior  surface 
of  the  eminence  into  a  canal  which  is  the  continuation  of 


258  PART  ir.      HISTOLOGICAL  ANATOMY. 

the  narrow  and  elongated  sinus;  while  the  development 
of  erectile  tissue  in  the  walls  of  the  canal  gives  rise  to  the 
corpus  spongiosum :  the  canal  becomes  the  urethra  of  the 
penis.  In  the  female  the  eminence  remains  small  and  be- 
comes the  clitoris :  the  sinus  becomes  short  and  broad  and 
is  represented  by  the  vestibule :  the  genital  folds  become 
the  labia  minores ;  the  genital  ridges  the  labia  majores. 

Glandular  diverticula  of  the  sinus  are  found  in  both 
sexes.  The  prostate  of  the  male  may  be  regarded  as  so 
derived;  there-is  no  corresponding  structure  in  the  female: 
the  glands  of  Cowper  and  the  glands  of  Bartholin  are  ho- 
mologous specializations  of  urethral  structures.  The 
change  of  position  of  the  gonads  has  been  already  re- 
ferred to.  Situated  at  first  in  the  more  anterior  region  of 
the  abdominal  portions  of  the  body  cavity,  they  undergo 
an  apparent  shifting  backward  in  both  sexes.  The  change 
is  least  in  the  female,  where  they  find  a  permanent  resting 
place  within  the  pelvis.  In  the  male  they  reach  the  brim 
of  the  pelvis,  penetrate  the  abdominal  wall,  and  push  their 
way  into  the  genital  ridges :  these  enlarge,  become  saccu- 
lar, and  coalesce  below  the  united  genital  fold  to  form 
the  scrotum. 


CHAPTER    XX.       VASCULAK    SYSTEM.  259 


CHAPTER  XX. 
THE  VASCULAR  SYSTEM. 

On  account  of  the  part  played  by  the  smaller  arteries, 
veins  and  lymphatics  as  components  of  the  organs  of  the 
body  in  which  they  occur,  the  structure  of  such  vessels  was 
described  in  one  of  the  earlier  chapters  of  this  book. 
The  larger  vessels  must  themselves  be  regarded  as  organs 
meriting  separate  consideration  equally  with  the  heart, 
which  is  the  central  organ  of  the  vascular  system. 

The  arteries  of  the  body  differ  most  conspicuously  from 
the  arterioles  found  in  the  various  organs  b}'  the  relative 
increase  of  the  media :  but  each  of  the  three  layers  under- 
goes both  increase  and  modification.  The  intima  is  lined, 
as  in  the  smaller  vessels,  by  endothelial  cells  elongated  in 
the  direction  of  the  tube :  but  this  endothelium  rests  upon 
a  subendothelial  layer  of  fibrous  tissue  with  branched 
corpuscles  that  disappears  as  we  pass  to  the  finer  subdi- 
visions of  the  vessel :  with  increase  in  the  size  of  the  artery 
it  becomes  more  fully  developed.  This  layer  is  directly  in 
contact  outwardly  with  the  elastic  layer,  which  is  well  de- 
veloped, especially  in  the  larger  arteries,  where  it  has  the 
form  of  a  fenestrated  membrane,  the  membrana  elastica 
intimae. 

The  media  consists  largeh^  of  transversely  disposed 
smooth  muscular  fibres,   particularly-  in  the  smaller  ar- 


260  PART    II.      HISTOLOGICAL    ANATOMY. 

teries  of  the  limbs:  in  large  arteries,  however,  there  is  a  dis- 
tinct admixture  of  elastic  tissue  in  the  form  of  a  network 
of  fibres;  this  is  connected  with  the  elastic  layer  of  the  in- 
tima,  and  pervades  the  whole  muscular  coat:  it  increases 
in  proportion  with  the  size  of  the  vessel.  The  muscular 
fibres  of  the  media  are  quite  short,  and  are  very  irregular 
in  form,  lacking  the  definite  spindle  shape  characteristic 
of  smooth  fibres  in  most  places  where  thej^  occur.  In  some 
of  the  larger  arteries  many  of  the  muscular  bundles  of  the 
inner  part  of  the  media  are  longitudinally  disposed. 

The  adventitia  is  the  stoutest  and  most  resistant  of  the 
coats  of  the  arteries.  It  is  rich  in  elastic  tissue,  especially 
toward  the  media,  the  larger  arteries  exhibiting  just  ex- 
ternal to  that  coat  a  distinct  elastic  la^^er,  the  membrana 
elastica  externa;  this  is  followed  by  a  region  rich  in  elas- 
tic fibres:  more  outwardl}^  the  adventitia  consists  almost 
wholly  of  closely  felted  bundles  of  white  fibrous  tissue 
w^hich  at  its  outer  surface  passes  over  into  the  intersti- 
tial areolar  tissue  found  between  the  vessel  in  question 
and  the  adjacent  organs.  In  some  of  the  larger  arteries 
longitudinal  bundles  of  smooth  muscular  fibres  are  found. 

In  connection  with  the  discussion  of  the  respiratory 
tract  regarded  as  a  diverticulum  of  the  alimentary  canal, 
it  was  pointed  out  that  the  coats  of  the  latter  were  desig- 
nated by  characters  readily  demonstrable  with  the  dis- 
secting knife,  but  that  the  proper  division  based  upon  his- 
tological characters  would  be  into  an  epithelial  and  a 
musculo-skeletal  la3'er.  The  coats  of  an  artery  as  above 
described  are  also  examples  of  structures  clearly  disting- 
uishable by  anatomical  methods :  but  a  consideration  of 


Cir.VPTF^R    XX.      VASCl'LAR    SYSTIvM.  261 

the  histology  of  the  wall  of  an  artery  will  show  the  cor- 
rectness of  the  description  that  has  been  proposed  for 
it  as  "composed  of  muscular  and  elastic  tissue  lined 
intcrnalh'  by  endothelium  and  strengthened  externally  by 
a  layer  of  connective  tissue." 

The  features  which  have  been  mentioned  as  character- 
istic of  the  largest  arteries  are  intensified  in  the  structure 
of  the  aorta.  In  it  the  elastic  layer  of  the  intima  is  not  so 
distinctly  membranous,  being  composed  chiefly  of  fibres 
which  pass  into  those  of  the  media,  the  two  coats  not  be- 
ing sharply  defined.  The  media  itself  not  only  contains  a 
ver}'  large  amount  of  elastic  tissue  in  proportion  to  the 
muscular  tissue  present,  but  is  also  reinforced  b}' a  consider- 
able quantit\^  of  white  fibrous  tissue,  the  coat  being  char- 
acterized more  especially  by  its  strength  and  elasticity 
than  by  its  contractility.  As  in  sojne  of  the  larger  arter- 
ies there  are  both  longitudinal  and  transvere  bundles  of 
muscular  fibres.  The  adventitia  is  not  sharply  defined 
from  the  media,  and  is  relatively-  thin.  The  pulmonary 
artery  agrees  in  most  respects  with  the  aorta  in  struct- 
ure. The  larger  arteries  and  veins  have,  like  other  or- 
gans of  the  body,  their  own  small  vessels  of  supply ;  these 
are  called  the  vasa  vasorum. 

While  the  heart  is  quite  a  complex  organ  from  the  ana- 
tomical standpoint,  its  histological  structure  is  compara- 
tively simple.  It  may  be  regarded  as  essentially  a  hollow 
mass  of  muscular  fibres  of  a  peculiar  sort,  having  on  its 
outside  an  investing  skeletal  laj-er,   and   a    similar  lin- 


262  PART   II.      HISTOLOGICAL   ANATOMY," 

ing  inwardl\,  each  of  the  layers  being  bounded  by  an  en- 
dothelium. The  outer  la\'er,  or  epicardium,  is  the  cardiac 
portion  of  the  pericardium.  It  is  invested  with  serous 
endothelium  which  rests  upon  a  membrane  whose  outer 
portion  consists  chiefl\^  of  white  fibrous  tissue,  but  which 
contains  in  its  deeper  layer  a  considerable  quantity  of 
elastic  fibres  arranged  in  a  loose  and  indefinite  meshwork. 
Beneath  the  membrane  is  a  stratum  of  areolar  tissue 
which  is  rich  in  fat  cells,  and  in  which  run  the  vessels  and 
nerves  of  the  wall  of  the  heart ;  in  the  auricles  there  are 
numerous  small  ganglia  connected  with  the  nerves.  The 
lining  membrane,  or  endocardium,  is  not  unlike  the  epi- 
cardium :  its  investing  endothelium,  while  continuous 
with  that  lining  the  arteries  and  veins,  and  agreeing  in 
function  with  vascular  endothelium,  is  composed  of  cells 
which  are  not  elongated  in  form,  but  resemble  rather  in 
outline  those  found  on  serous  surfaces.  The  membrane 
contains  a  few  muscular  fibres,  and  the  subjacent  con- 
nective tissue  but  a  limited  quantity  of  fat  cells.  In  the 
auricles  the  elastic  tissue  of  the  membrane  is  quite  well 
developed. 

The  myocardium  or  muscular  layer  of  the  heart  varies 
greatlv  in  thickness,  being  most  developed  in  the  ventricu- 
lar wall  and  least  in  some  portions  of  the  auricles.  The 
peculiar  cardiac  muscular  elements  of  which  it  is  com- 
posed have  already  been  described :  they  are  arranged  in 
bundles  which  form  the  fibres  and  lamellae  visible  to  the 
naked  eye:  the  disposition  of  the  latter  is  a  subject  of  an- 
atomical rather  than  histological  study.  Between  the 
bundles  is  a  very  delicate  framework  of  connective  tissue 


CHAPTER   XX.   VASCULAR   SYSTEM.  263 

which  supports  the  abundant  blood  and  lymph  capillar- 
ies: patches  of  adenoid  tissue  are  also  of  occasional  occur- 
rence. The  peculiar  fibres  of  Purkinje  found  just  be- 
neath the  endocardium  of  some  mammals  are  of  doubtful 
occurrence  io  man:  they  are  large  cellular  elements  (fre- 
quently with  two  nuclei)  whose  central  portion  consists 
of  clear  protoplasm  but  whose  surface  has  undergone 
striation ;  they  may  perhaps  be  regarded  as  imperfectly 
developed  cardiac  fibres. 

The  columnae  carneae  and  the  papillary  muscles  are 
alike  processes  of  the  myocardium :  in  them  the  muscular 
fibres  are  chiefly  disposed  in  a  longitudinal  direction,  and 
the  endocardial  membrane  upon  their  surfaces  becomes 
almost  tendinous  in  structure;  the  latter  passes  over  from 
the  surface  of  the  papillary  muscles  into  the  chordae  ten- 
dineae,  whose  central  strands  of  fibrous  tissue  are  in  the 
larger  cords  reinforced  by  scattered  bundles  of  muscular 
fibres. 

The  valves  of  the  heart  are  folds  of  the  endocardium 
containing  bundles  of  fibrous  tissue  and  a  variable  quan- 
tity' of  elasjtic  tissue,  the  latter  being  present  in  greatest 
quantity  in  those  places  subject  to  the  greatest  pressure 
produced  by  the  heart's  action  on  the  blood :  the  semi- 
lunar valves  are  farther  reinforced  by  the  presence  in  each 
of  a  nodule  composed  largely  of  elasticjtissue,  the  corpus 
arantii. 

The  large  veins  differ  from  the  corresponding  arteries  in 
the  relative  thinness  of  the  media  and  the  thickness  of  the 
adventitia,  and  in  the  smaller  amount  of  elastic  tissue 
throughout  the  entire  structure.    The  endothelial  cells  lin- 


264*  PART   11.      HISTOLOGICAL   ANATOMY. 

ing  the  intima  of  the  larger  veins  are  but  slightly  if  at  all 
elono^ated,  resembling  those  of  the  endocardium  in  their 
polygonal  outline:  in  the  smaller  tributaries  the  form  char- 
acteristic of  vascular  endothelium  is  assumed.  The  media 
of  the  two  venae  cavae  is  a  continuation  of  the  mj^ocar- 
dium  of  the  right  auricle,  and  contains  cardiac  muscular 
elements  :  that  of  the  larger  veins  generally  contains  but  a 
small  quantity  of  muscular  tissue,  intermingled  with 
bundles  of  white  fibres,  which  replace  the  elastic  tissue 
similarly  situated  in  the  larger  arteries.  The  media  of 
some  veins,  notably  those  of  the  nervous  axis  and^of  the 
bones,  is  entirely  devoid  of  muscular  tissue.  In  a  few- 
cases  the  muscular  bundles  situated  in  the  inner  part  of 
the  media  are  longitudinal  in  their  direction.  The  adven- 
titia  of  the  veins  consists  chiefly  of  a  stout  layer  of  felted 
bundles  of  fibrous  tissue,  containing  a  moderate  amount 
of  elastic  fibres.  In  the  portal,  renal,  and  some  other  veins, 
the  adventitia  contains  numerous  bundles  of  smooth 
muscular  fibres  longitudinally  disposed.  The  valves  of 
the  veins  are  folds  of  the  intima,  strengthened  by  bundles 
of  fibrous  tissue  arranged  parallel  to  their  free  borders, 
and  containing  a  small  amount  of  elastic  tissue:  mus- 
cular bundles  from  the  media  sometimes  extend  into  the 
base. 

The  lymphatic  vessels  resemble  the  blood  vessels  in  the 
fact  that  their  walls  are  composed  of  an  inner,  a  middle, 
and  an  outer  coat :  each  of  the  three  coats  is,  however, 
much  thinner  than  in  the  case  of  the  blood  vessels,  the 
middle  and  outer  being  especially  so.    It  is  customary  to 


CHAPTER   XX.      VASCULAR  SYSTEM.  265 

say  that  they  resemble  the  veins  in  structure:  it  is  more 
accurate  to  say  that  in  the  relative  thickness  of  the  coats 
and  particularly  in  the  proportion  of  muscular  tissue  they 
approach  the  arteries :  they  resemble  the  veins  in  the 
structure  of  the  intima  and  in  its  development  into  valvu- 
lar folds :  the  adventitia  is  relatively  to  the  other  coats 
weaker  and  simpler  than  in  either  arteries  or  veins.  This 
is  not  the  case,  however,  with  the  most  highly  specialized 
of  all  the  lymphatic  vessels,  the  thoracic  duct,  whose  ad- 
ventitia is  quite  stout  and  contains  longitudinal  bundles  of 
smooth  muscular  fibres :  the  intima  of  the  duct  is  also  re- 
inforced with  a  longitudinal  network  of  elastic  fibres.  In 
the  smaller  lymphatic  vessels  the  adventitia  disappears; 
the  media  contains  bundles  of  smooth  muscular  fibres 
obliquely  disposed,  in  addition  to  the  ordinary  transverse 
bundles. 

The  development  of  the  heart  and  of  the  larger  vascular 
trunks  takes  place  in  such  a  manner  that  it  is  difficult  to 
discuss  it  without  either  taking  for  granted  a  knowledge 
of  or  entering  into  a  description  of  the  manner  in  which 
the  ground  plan  of  the  embryo  body  is  laid  down.  It  is 
therefore  necessary  to  defier  it  until  after  the  subject  of 
Embryology  has  been  taken  up  by  the  student. 


Reference  mav  with  convenience  at  this  time  be  asrain 
made  to  the  great  serous  membranes  which  line  the 
principal  cavities  of  the  body ;  since  these  latter  are  best  re- 


266  PART    II.      HISTOLOGICAL    ANATOMY. 

garded  as  enormous  lymph  cavities.  Their  general  struct- 
ure and  particularlj^  that  of  their  characteristic  epithe- 
lium has  been  discussed  in  a  previous  chapter:  it  remains 
to  speak  briefly  of  some  of  the  features  characteristic  of 
each  (in  so  far  as  they  have  distinguishing  characteristics) 
and  to  describe  some  allied  though  different  structures 
usuall}^  associated  with  them. 

The  right  and  left  pleurae,  which  invest  the  lungs,  line 
the  thoracic  cavities  and  bound  the  mediastinum  which 
separates  the  latter,  while  they  have  the  general  charac- 
ter of  serous  membranes,  vary  somewhat  in  structure  in 
different  localities.  They  are  thickest  over  the  ribs,  where 
the  subserous  layer  of  areolar  tissue  permits  of  the  ready 
removal  of  the  membrane :  the  stomata  of  the  endothe- 
lium are  said  to  occur  over  the  intercostal  spaces  onlv-.  The 
membrane  becomes  thinner  and  more  adherent  as  it  passes 
onto  the  surface  of  the  diaphragm.  On  the  surface  of  the 
lungs  it  is  quite  thin  and  closely  attached,  the  connective 
tissue  becoming  continuous  with  that  of  the  framework 
of  the  organ:  the  endothelial  cells  of  the  pulmonary 
pleura  are  also  taller  and  more  granular.  The  pericardium, 
or  heart  sac,  is  in  reality  but  little  more  than  a  double 
serous  membrane:  its  outer  portion  being  derived  from  the 
pleurae  and  its  inner  forming  the  proper  pericardial  mem- 
brane which  is  reflected  upon  the  heart  as  the  epicardium : 
between  the  two  membranes  is  a  layer  of  areolar  tissue 
containing  a  small  quantity  of  fat  and  numerous  |blood 
vessels  and  lymphatics. 

The  peritoneum,  which  lines  the  cavity  of  the  abdomen, 
is  the  largest  and  most  complicated  of  the  serous  mem- 


CHAPTER   XX.      VASCULAR  SYSTEM  267 

branes.  Like  the  pleura,  it  is  thicker  in  its  parietal  than 
its  visceral  portion.  The  subserous  layer  of  areolar  tis- 
sue is  less  developed  along  the  ventral  midline  and  on  the 
under  surface  of  the  diaphragm  than  elsewhere  in  the 
parietal  peritoneum,  which  is  consequent!}'  most  closely 
adherent  in  these  regions :  that  of  the  visceral  portions  is 
in  every  case  closely  connected  with  the  skeletal  structures 
of  the  organs  invested.  Certain  of  its  folds,  termed  liga- 
ments, from  their  mechanical  relations  to  the  organs  with 
which  they  are  connected,  for  example,  the  spleen,  the  liver, 
and  the  uterus,  have  their  applied  fibrous  membranes  in 
close  contact.  Others,  such  as  the  omenta,  have  between 
the  membranes  a  laj'er  of  areolar  tissue  in  which  a  greater 
or  less  amount  of  fat  may  be  developed.  In  the  me- 
sentery the  intervening  layer  contains,  in  addition  to  a 
variable  amount  of  fat,  an  extensive  system  of  blood  and 
Ij'mph  vessels:  associated  with  the  latter  are  the  numer- 
ous nodules  of  adenoid  tissue  commonly'  called  the  lacteal 
glands :  their  structure  will  be  described  in  the  following 
chapter  in  connection  with  other  similar  bodies.  The 
tunica  vaginalis  of  the  scrotum  and  testis  must  be  re- 
garded as  an  oft'set  from  the  peritoneum :  its  structure 
and  relations  have  been  sufficiently  described  in  a  pre- 
vious chapter. 

The  meninges  of  the  brain  and  spinal  cord,  and  in  par- 
ticular the  pia  mater,  have  been  regarded  as  serous  mem- 
branes :  the}'  differ,  however,  in  structure  and  relations 
alike  from  the  ordinary  membranes  of  that  name,  and  will 
best  be  discussed  in  connection  with  the  nervous  axis.  The 
perilymphatic  and  endolymphatic  surfaces  of  the  internal 


268  PART   II.    HISTOLOGICAL   ANATOMY. 

ear  will  be  considered  in  connection  with  the  description  of 
that  organ. 


The  synovial  membranes,  which  line  the  capsules  of 
joints,  the  sheaths  of  tendons,  etc.,  may  here  be  consid- 
ered, although  they  differ  materially  in  structure  from  the 
serous  membranes  wnth  which  they  are  often  associated 
as  regards  both  their  organization  and  also  the  charac- 
teristic fluid  which  fills  the  cavities  bounded  by  them. 
They  are  in  effect  rather  dense  membranous  layers  of  con- 
nective tissue,  devoid  of  any  well-defined  endothelial  in- 
vestment :  here  and  there  cells  and  patches  of  cells  may  be 
seen  upon  their  surfaces,  some  of  which  are  sufficiently 
close  together  to  assume  the  polygonal  outline  character- 
istic of  endothelium;  others,  how^ever,  are  distinctly 
branched,  differing  in  no  essential  from  ordinary  connect- 
ive tissue  corpuscles :  their  affinity  to  endothelium  will 
perhaps  be  evident  if  we  recall  the  definition  of  it  previ- 
ousl}^  given  to  the  effect  that  it  is  a  layer  of  connective 
tissue  corpuscles  investing  a  free  surface.  Fringed  vascu- 
lar folds  occur  upons\movial  membranes  which  frequently 
bear  smaller  processes,  the  synovial  villi,  in  which  a  cen- 
tral strand  of  fibrous  tissue  is  invested  with  a  layer  of 
small  rounded  cells.  Articular  synovial  membranes  pass 
so  gradually  into  the  fibro-cartilage  which  borders  the  true 
articular  cartilage  of  the  joint  that  it  is  not  possible  to 
define  the  boundary  between  the  two :  even  the  cellular 
elements  gradually  losing  their  processes  and  presenting 
the  appearance  of  cartilage  corpuscles. 


CHAPTER    XXI.      THE    DUCTLESS   BODIES.  269 


CHAPTER  XXI. 

THE   DUCTLESS  BODIES. 


The  organs  here  included  under  the  above  title  are  very 
frequently  referred  to  by  the  name  of  the  ductless  glands. 
If  the  term  gland  be  used  in  its  older  and  looser  sense  as 
a  designation  for  any  soft  parench3'matous  body,  then  a 
distinction  between  such  of  those  bodies  as  are  provided 
with  and  those  devoid  of  ducts  is  natural  and  justifiable. 
It  is  better,  however,  to  use  the  term  in  its  narrower  and 
more  definite  application  to  bodies  composed  essentially 
of  acini  or  tubules  lined  with  epithelial  cells  whose  funct- 
tion  is  to  secrete  a  specific  solution  or  soluble  substance, 
ordinarily  removed  through  a  definite  channel  of  dis- 
charge. If  the  term  gland  be  thus  employed,  there  is  but 
one  (and  a  portion  of  another)  of  the  bodies  here  consid- 
ered to  which  the  expression  ductless  gland  may  with  any 
propriety  be  applied :  as  used  to  designate  the  others  it  is 
not  only  erroneous,  but  tends  to  give  rise  to.  misleading 
conceptions.  The  expression  ductless  bodies,  while  not 
without  objections,  is  therefore  preferable  to  the  older 
title.  The  name  of  adenoid  bodies  has  also  been  pro- 
posed, but  is  apt  to  lead  to  the  false  idea  of  a  community 
of  structure  in  all  of  the  bodies  in  question. 

It  is  quite  important  to  note  in  this  connection  that  the 
caption  given  to  this  chapter,  unlike  any  other  used  hith- 


1'  ^ 


\ 

^5^70  PART  n.  HISTOLOGICAL  ANATOMY.  . 

erto,  does  not  designate  any  s\'stem  of  organs  associated 
with  the  performance  ofone  of  the  great  functions  or  groups 
of  functions,  or  characterized  bv  anv  common  structure 
or  b}' similar  anatomical  or  morphological  relations :  their 
association  under  one  heading  is  more  than  anything  else 
a  matter  of  convenience ;  and  the  order  of  their  consider- 
ation largely  arbitrary,  though  not  altogether  so.  The 
lymphatic  bodies,  from  their  close  structural  and  func- 
tional relation  to  the  Ij'mphatic  vessels,  might  with  equal 
propriety  be  described  in  connection  with  the  vascular  sys- 
tem :  the  spleen,  differing  in  important  respects  from  the 
bodies  just  named,  resembles  them  in  its  close  relation  to 
the  circulatory  system  :  the  thymus  has  much  in  common 
w^ith  them  in  its  structure:  to  all  these  bodies  (and  to  these 
alone)  the  term  adenoid  bodies  might  with  propriety  be 
applied,  owing  to  the  prevalence  in  them  of  the  tissue  of 
that  name. 

The  thymus,  in  addition  to  the  adenoid  tissue  of  which 
it  is  largely  composed,  contains  definite  masses  of  epithe- 
lial elements  whose  derivation  will  be  discussed  when 
that  body  is  described :  the  thyroid,  is  largely  composed 
of  epithelial  cells  which  line  acini ;  to  it  more  than  to  any 
other  of  the  bodies  here  considered  may  the  title  of  duct- 
less gland  be  with  propriety  applied,  as  is  shown  b}^  its 
embryonic  development.  Resembling  the  thymus  and 
the  thyroid  in  the  origin  of  some  of  their  elements  (and  in 
little  else)  are  the  parathyroids,  situated  on  either  side  of 
each  lobe  of  the  thyroid ;  and  the  carotid  glands ;  they 
are  closely  resembled  in  structure,  though  not  in  origin, 
by  the  coccygeal  gland;  the  term  gland  is  still  univer- 


CHAI'TKK    XXI.      THE   DUCTLESS   BODIES.  271 

sally  applied  to  the  last  two;  they  have  been  called,  from 
their  associations,  the  arterial  glands. 

The  suprarenal  capsules,  or  the  adrenals,  as  they  are 
sometimes  termed,  are  double  structures,  consisting  in 
part  of  a  mass  of  nervous  tissue:  they  are  resembled  in 
this  respect  by  the  pituitary  body,  which  consists  in  part 
of  a  mass  somewhat  analogous  to  the  thyroid,  and  in 
part  of  an  atrophied  lobe  of  the  brain:  the  pineal  body 
is  altogether  derived  from  the  modification  of  a  portion 
of  the  brain,  and  might,  in  common  with  the  pituitary 
body,  be  described  in  connection  with  that  organ.  This  brief 
enumeration  will  suffice  to  show  how  heterogenous  are 
the  bodies  here  associated  as  regards  both  structure  and 
anatomical  relations:  the  function  of  most  of  them  is  at 
present  entirely  unknown.  An  account  of  the  details  of 
their  structure  will  now  be  given. 

The  simplest  lymphatic  bodies  are  those  rounded  masses 
of  adenoid  tissue  bounded  by  a  more  or  less  definite  fib- 
brous  layer  which  have  already  been  mentioned  in  con- 
nection with  the  description  of  the  alimentary  canal.  The 
lymphatic  follicles,  as  they  are  commonly  called,  of  the 
intestine,  whether  solitary  or  clustered  (as  in  the  Peyer's 
patches  of  the  ileum),  and  the  similar  masses  whose  some- 
what confluent  aggregations  form  the  tonsils,  are  exam- 
ples of  what  may  perhaps  best  be  called  lymphatic  nod- 
ules. They  consist  essentiall}^  of  spheroidal  lumps  of  ade- 
noid tissue  somewhat  more  dense  in  the  outer  than  the 
inner  portion,  but  showing  no  division  into  lobes.  Thej' 
are  pervaded  by  capillary  networks,  and  may  be  invested 


272  PART  II.      HISTOLOGICAL  ANATOMY. 

by  a  large  lymph  sinus  or  by  a  plexus  of  small  lymphatic 
vessels.  It  is  not  easy  to  demonstrate  among  the  latter 
distinctl}^  efferent  or  afferent  trunks :  and  from  the  usual 
position  of  these  bodies  in  mucous  membranes,  and  from 
what  we  now  know  of  the  important  bacteriophagous 
function  of  leucocytes  upon  mucous  surfaces  it  may  be 
questioned  whether  those  formed  in  the  lymphatic  nodules 
do  not  find  their  chief  destination  there  rather  than  in  the 
l3''mph  vStreara. 

The  larger  and  more  deeply  seated  h^mphatic  bodies 
commonly  termed  lymphatic  glands,  together  with  the 
altoofether  similar  bodies  found  in  the  mesentery  and  called 
lacteal  glands,  for  both  of  which  the  much  better  name 
of  lymphatic  nodes  has  of  late  years  come  into  use,  are 
in  reality  integral  parts  of  the  lymphatic  and  therefore  of 
the  circulatory  system.  Each  is  a  rounded  body  showing 
a  distinct  depression,  the  hilum,  on  one  side,  at  which 
point  the  blood  vessels  of  the  interior  enter,  and  from 
which  one  or  more  efferent  lymphatics  leave  the  organ. 
The  surface  is  invested  with  a  stout  fibrous^capsule,  con- 
taining scattered  bundles  of  smooth  muscular  fibres  in  the 
larger  nodes:  it  has  a  proper  network  of  blood  vessels, 
and  into  it  at  various  points  pass  afferent  lympahtics 
which  traverse  it  quite  obliquely,  sometimes  forming 
small  plexuses  or  sinuses  within  it  before  communica- 
ting with  the  lymph  channels  within. 

The  interior  is  divided  into  a  cortical  and  a  medullary 
portion :  from  the  capsule  stout  trabeculae  of  fibrous  tis- 
sue (sometimes  containing  smooth  muscular  fibres)  pass 
inward :  they  are  frequently  broad  and  lamellar  in  form 


CHAPTER   XXI.   THE   DUCTLESS  GLANDS.  273 

and  divide  the  cortex  imperfectly  into  suljecjual  lobules 
sometimes  called  the  cortical  follicles  and  compared  to 
the  lymphatic  nodules  (or  so-called  follicles)  already  de- 
scribed, from  which,  however,  they  differ  in  important  re- 
spects: the  inner  extremities  of  their  trabeculae  subdivide, 
communicating  by  their  branches  with  a  coarse  network 
of  fibrous  tissue  which  forms  the  framework  of  the  medul- 
larx'  portion. 

The  spaces  of  the  network  just  mentioned  are  occupied 
b\^  another  network  of  rounded  strands  of  adenoid  tis- 
sue, the  medullary  cords,  which  become  continuous  at 
their  extremities  with  the  solid  cortical  lobules  of  the 
same  tissue  situated  between  the  trabeculae.  It  will  be 
remembered  that  adenoid  tissue  ^nsists  in  effect  of  reti- 
form  tissue  whose  interstices  are  filled  wrth  lymphoblasts: 
the  medullary  cords  and  cortical  lobules  of  adenoid  tissue 
are  in  each  case  smaller  than  the  cavities  in  which  they  are 
situated,  the  surrounding  spaces  containing  a  coarser  ret- 
iform  tissue  (devoid  of  lymphoblasts  and  presenting  but 
little  resistance  to  the  passage  of  fluids)  which  connects 
the  cords  and  lobules  with  the  medullary  and  trabecular 
framework.  A  network  of  passages  throughout  the  whole 
node,  known  as  the  lymph  sinuses  or  lymph  channels, 
is  thus  formed,  into  which  the  afferent  l^'mphatics  open 
after  passing  through  the  capsule,  and  from  which  the 
efferent  lymphatics  lead. 

The  medullary  portion  of  the  lymphatic  node  extends  to 
the  surface  at  the  hilum,  the  artery  of  supply  entering  it 
directly :  the  capillary  network  of  the  interior  is  situated 
almost  entirely  in  the    deeper    portion    of   the    adenoid 


,>f 


274  PART  n.      HISTOLOGICAL  ANATOMY. 

strands  and  masses.  The  lymph  channels  of  the  medulla 
converge  at  the  hilumtororm  a  plexus,  from  which  a  single 
efferent  trunk  may  lead,  or,  in  the  larger  nodes,  several 
smaller  vessels  which  unite  outside  the  node  to  form  a 
single  trunk.  Pacinian  bodies  are  of  frequent  occurrence 
in  the  interstitial  connective  tissue  j  ust  without  the  ^ap- 
sjale  at  the  hiluni. 

The  relation  between  the  lymphatic  nodes  and  the  lymph 
stream,  as  regards  the  formation  of  lymphocytes  and 
their  transformation  in  the  blood  stream  into  leucocytes, 
has  been  discussed  in  a  previous  chapter.  The  minute  is- 
lands of  adenoid  tissue  occasionally  found  either  on  or 
within  the  walls  of  lymphatic  vessels,  and  known  as  peri- 
lymphatic or  endolymphatic  nodules,  may  be  regarded 
as  rudimentary  organs  of  the  same  kind. 

The  spleen  is  as  closely  related  to  the  blood  vascular  as 
the  lymphatic  nodes  to  the  lymphatic  portion  of  the  cir- 
culatory system :  it  may  possibly  be  regarded  as  derived 
from  the  modification  of  one  of  the  last  named  bodies, 
though  differing  from  them  greatly  not  only  in  its  vascu- 
lar relations,  but  also  in  its  internal  structure.  It  is  inter- 
esting to  note  that  in  some  of  the  lower  mammals  num- 
erous small  accessory  spleen-like  nodules  are  norrnally  pre- 
sent in  other  regions  of  the  body.  Similar  bodies  are 
sometimes  found  in  man  in  the  vicinity  of  the  principal 
organ,  of  which  they  may  be  regarded  as  diverticula. 

The  spleen  is  invested  by  a  serosa  derived  from  the  peri- 
toneum which  rests  upon  a  stout  capsule  of  fibrous  tissue 
which,  like  that  of  the  lymphatic  nodes,  contains  occa- 


CHAPTER   XXI.   TIIK   DUCTLESS  GLANDS.  275 

sional  sraqoth_muscular  fibres:  it  differs  from  that  of  the 
bodies  last  mentioned  in  the  greater  predominence  of  elas- 
tic fibres,  making  the  organ  highly  distensible.  At  the 
hilum  the  capsule  is  continued  into  the  spleen  to  form  large 
trabeculae. which  branch  and  subdivide  within,  eventually 
becoming  continuous  with  the  branches  of  similar  though 
small  trabeculae  which  pass  inward  from  the  capsule  at 
numerous  points:  the  trabeculae,  like  the  capsule,  contain 
numerous  elastic  fibres,  and  some  muscular  fibres :  the 
interior  of  the  organ  is  pervaded  by  the  large-meshed  re- 
ticular framework  thus  produced.  Continuous  with  this 
framework  is  a  coarse  retiform  tissue  whose  fibrous  net- 
work is  invested  with  branched  corpuscles :  in  many  cases 
the  fibrous  element  is  quite  scanty,  and  the  reticulum  con- 
sists of  little  more  than  branched  corpuscles  connected 
with  each  other  by  the  tips  of  their  branches,  the  so-called 
reticular  cells  of  the  spleen.  The  intervals  between  these 
cells  are  filled  with  blood  which  contains  rather  more  than 
the  usual  proportion  of  colorless  corpuscles  and  rather  less 
of  the  colored :  there  are  present  also  numerous  unbranch- 
ed  amoeboid  cells  somewhat  larger  than  colorless  corpus- 
cles, the  spleen-cells:  these,  the  reticular  cells,  and  the 
^asma  of  the  blood  itself  contains  disintegrating  colored 
corpuscles,  and  pigment  granules  derived  therefrom :  the 
whole  constitutes  the  spleen  pulp,  a  reddish  brown 
mass  to  which  the  characteristic  color  of  the  organ  is  due. 
The  splenic  arterj'  divides  into  several  branches  just  be- 
fore reaching  the  organ  :  these  enter  at  the  hilum,  follow- 
ing the  stout  trabecular  continuations  ofthe  capsule  above 
mentioned :  within  the  latter  they  branch,  their  branches 

^^'"'^"         f     i      J  .,       r      L     , 


/luJ.^u^u^^Mf^'^^ 


276  PART  n.   HISTOLOGICAL  ANATOMY. 

in  some  cases  followmg  the  subdivisions  of  the  trabeculae. 
In  other  cases  small  branches  leave  the  trabeculae  and  be- 
come divided  into  brush-like  tufts  of  arterioles:  on  emerging, 
their  adyentitia,  heretofore  continuous  with  the  fibrous 
tissue  of  the  framework  of  the  organ,  becomes  replaced 
by  a  layer  of  adenoid  tissue :  here  and  there  this  sheath  is 
suddenh'  enlarged  to  form  spheroidal  masses  which  ma}' 
be  as  much  as  a  millimetre  in  diameter,  though  usually 
less  than  half  as  large:  thej^are  known  as  the  Malpighian 
corpuscles  of  the  spleen,  and  are  readily  visible  to  the 
naked  e^-e  as  whitish  spots  in  the  dark  brown  pulp.  The 
adenoid  tissue  of  the  corpuscles  is  permeated  b}'  capillaries 
given  off  from  the  arterioles  enclosed ;  it  is  quite  loose  in 
the  centre  but  denser  at  the  surface,  where  it  passes  over 
abruptly  into  the  retiform  tissue  of  the  pulp.  The  cor- 
puscles are  found  surrounding  small  arteries  and  appar- 
ently strung  upon  them,  or  upon  their  subdivisions,  in 
which  case  they  look  like  lateral  outgrowths. 

fOn  leaving  the  corpuscles  the  arteries  divide  into  capil- 
laries, which,  like  those  arising  from  the  smaller  divisions  of 
the  arteries  which  follow  the  framework  of  the  spleen  more 
1     closely,  finall}^  open  into  the  spaces  of  the  tissue  of  the 
_y    pulp,  the  endothelial  cells  of  the  capillaries  gradually  be- 
1   coming  looser,  branching,  and   finally  passing  over  into 
I )  the  reticular  cells ;  thus^f|m:din^  the  only  instance  jn  the 
body  where  the  blood  leaves  the  definite  vessels  ,prc)ger_to 
\\it  and  circulates  in  the  interstices  of  the  tissues  ;  a  condi- 
tion largely  characteristic  of  the  circulation  of  all  inver- 
tebrates.   The  veinlets  of  the  pulp  originate  in  the  same 
way  that  the  arterioles  terminate,  or  rather  its  converse : 


-;Uu^CMf>4^ 


CHAPTER    Xxi.    THE  DUCTLESS  GLANDS.  277 

the  reticular  cells  passing  over  into  branched  and  loosely 
disposed  endothelial  cells  which  later  become  closely  united 
to  form  the  lining  of  the  commencing  vessels.  The  latter 
soon  enter  the  trabeculae,  where  they  are  gathered  into 
larger  vein&j  these  anastomose  freely  within  the  trabecu- 
lae, finalh'  uniting  to  form  the  few  large  trunks  that  leave 
the  hilum. 

The  thymus  is  a  bijobed  adenoid  body  situated  just  be- 
neath the  sternum  in  the  upper  part  of  the  thorax  and  ex- 
tending into  the  lower  part  of  the  neck  in  the  embryo  and 
the  infant:  it  is  gradually  reduced  to  a  mere  vestige  in  the 
adult.  The  whole  organ  is  invested  by  a  thin  capsule  of 
fibrous  tissue,  beneath  which  it  is  subdivided  into  a  num- 
ber of  irregular  lobules  each  but  a  few  millimetres  in  di- 
ameter. The  fibrous  tissue  envelope  of  each  lobule  gives 
off  trabeculae  which  penetrate  the  interior  in  the  same 
manner  as  the  similar  structures  in  a  13'mphatic  node.  The 
lobule  in  consequence  exhibits  a  cortical  and  a  medullarv 
portion.  The  cortex  is  composed  of  nodules  of  adenoid 
tissue  not  unlike  those  found  in  a  tonsil :  the  medulla  is  a 
mass  of  adenoid  tissue  much  less  dense  than  that  of  the 
cortex,  the  transition  from  the  one  to  the  other  being  so 
rapid  as  to  be  quite  conspicuous  in  sections.  The  trabec- 
ulae are  continuous  with  the  retiform  tissue  of  the  cortex 
and  medulla  alike,  but  there  is  no  distinct  raedullar\' frame- 
\vork  of  fibrous  tissue  and  no  segregation  of  the  adenoid 
tissue  in  medullary  cords:  nor  are  there  any  lymph  chan- 
nels in  either  cortex  or  medulla. 

The  most  characteristic  feature^  of  the  thymus  is  the 


- —  CJUa^iU^'  f 

278         --      PART  II.     HISTOLOGICAL  ANATOMY. 

presence  in  the  medulla  of  what  are  known  as  Hassall's 
corpuscles,  or,  as  that  histologist  termed  them,  concen- 
tric corpuscles.  These  are  peculiar  nest-like  groups  of  epi- 
thelioid cells  which  are  now"  known  to  be  derived  from  the 
breaking  up  of  right  and  left  tubular  diverticula  from  the 
cervical  hypoblast.  Each  corpuscle  consists  of  a  central 
granular  mass  containing  one  or  more  spheroidal  cells, 
surrounded  by  two  or  three  layers  of  concentric  flattened 
cells :  compound  corpuscles  sometimes  occur,  two  or  three 
ordinary  corpuscles  being  invested  by  a  common  layer  of 
concentric  cells.  The  adenoid  tissue  of  the  organ  contains 
a  rich  capillary  network,  and  is  the  place  of  origin  of  nu- 
merous large  lymphatics. 

The  thyroid  resembles  the  thymus  in  being  relatively 
large  in  foetal  life  and  infancy :  it  differs  from  that  organ 
in  its  persistence  and  evident  functional  importance  in  the 
adult,  as  indicated  by  the  grave  consequences  of  its  com- 
plete extirpation.  Like  the  tubular  structures  which  even- 
tually break  up  into  the  concentric  corpuscles  of  the  thymus, 
its  characteristic  elements  are  derived  from  the  cervical 
hypoblast.  It  is  at  first  provided  with  a  duct,  whose  rudi- 
ment becomes  the  foramen  caecum  of  the  dorsum  of  the 
tongue:  the  duct  in  question  in  rare  instances  persists; 
in  the  great  majority  of  cases  it  aborts,  converting  the 
organ  into  a  true  ductless  gland. 

Unlike  most  glandular  bodies,  the  thyroid  is  not  pro- 
vided with  a  well-defined  capsule.  It  is  invested  by  a  layer 
of  areolar  tissue  considerably  denser  than  that  connecting 
it  with  adjacent  organs  but  not  passing  over  into  a  dis- 


CHAPTER   XXI.    THE   DUCTLESS  GLANDS.  279 

tinct  fibrous  ineinl)ranc.  Areolar  tissue  of  variable  density 
])erva(k's  the  interior  of  the  organ,  forming  the  support- 
ing framework  of  its  structure,  the  characteristic  feature 
of  which  is  the  jiresence  of  great  numbers  of  vesicles 
united  into  imperfect  lobules.  The  vesicles  are  spheroidal, 
polyhedral,  or  sometimes  tubular  in  form,  their  walls  con- 
sisting of  a  single  laver  of  cubqi^al  epithelium.  The  inte- 
rior of  the  vesicle  is  filled  with  a  glairy  yellowish  colloid 
substance  which  frequently  contains  leucocytes  and  de- 
tached epithelial  cells.  A  distinction  has  been]  made  by 
some  observers  between  the  colloid,  cells,  which  are  act- 
ivelv  engaged  in  secreting  the  fluid  contained  in  the  ves- 
icles, and  the  reserve  cells.  A  definite  basement  mem- 
brane cannot  be  clearly  discerned,  the  epithelial  cells  ap- 
pearing to  rest  directly  upon  the  interstitial  septa  of  are- 
olar tissue  already  mentioned :  the  areolae  of  the  septa 
not  infrequently  contain  the  colloid  secretion  of  the  cells: 
elements  resembling  plasma  cells  are  found  in  the  inter- 
stitial tissue.  The  thyroid  is  highly  vascular,  the  arteries 
being  relativel}'  quite  large,  and  anastomosing  freely :  the 
vesicles  are  surrounded  by  a  rich  capillar}'  network :  the 
lymphatics  are  also  large  and  numerous,  and  the  presence 
of  colloid  substance  in  their  interior  may  sometimes  be 
detected. 

Imbedded  in  the  substance  of  the  thyroid  upon  both 
the  lateral  and  the  mesial  surfaces  of  the  lobes  are  small 
bodies  a  few  millimeters  in  diameter  to  which  the  name  of 
parathyroids  has  been  given.  They  resemble  the  thyroid 
in  color  ^ind  appearance,  but  differ  from  it  in  ^structure, 


f';/^/ 


280  PART    n.      HISTOLOGICAL    ANATOMY. 

consisting  of  solid  strands  of  epithelioid  cells  apparently 
anastomosing,  their  interspaces  being  occupied  by  numer- 
ous blood  vessels.  They  have  been  regarded  by  some  ob- 
servers as  masses  of  embryonic  thyroid  tissue:  this  view- 
is  denied  by  others,  who  regard  their  structure  as  more 
nearly  approaching  that  of  the  carotid  glands.  There  is 
usually  associated  with  each  a  small  mass  of  adenoid  tis- 
sue containing  concentric  corpuscles  and  in  other  respects 
resembling  the  substance  of  the  thymus. 

The  carotid  glands,  situated  in  the  angle  between  the 
branches  of  the  common  carotid  artery,  are  small  irregu- 
larly shaped  bodies  whose  envelope  of  connective  tissue  is 
continued  inward  to  form  a  supporting  framework  im- 
bedded in  which  are  nodular  masses  of  epithelioid  cells 
richly  supplied  with  capillaries.  They  resemble  the  para- 
thyroids in  their  origin  from  the  cervical  hypoblast,  and 
both  are  probably  to  be  regarded  as  rudiments  of  larger 
and  more  important  organs. 

The  coccygeal  gland  is  another  body  of  quite  similar 
structure  to  those  just  described,  and  probably  also  rudi- 
mentary in  its  character.  The  epithelioid  cells  which  con- 
stitute its  distinguishing  feature  are  to  some  extent  dis- 
posed in  columnar  strands  as  in  the  parathyroids.  Eberth 
has  described  among  them  nests  of  cells  resembling  con- 
centric corpuscles.  Attempts  have  been  made  to  show 
that  the  elements  of  this  body  are  largely  nervous  in  char- 
acter, but  this  view  of  their  nature  lacks  confirmation. 
The  mode  of  development  is  not  known. 


CHAPTER   XXI.    THE   DUCTLESS  GLANDS.  281 

The  suprarenal  capsules,  or,  in  the  lan<2:uage  of  com- 
parative anatomy,  the  adrenal  bodies,  since  their  position 
is  usually  near  but  not  upon  the  kidneys,  as  is  the  case  in 
man,^  are  in  some  respects  the  most  complex  in  structure 
of  all  the  -ductless  bodies,  to  no  other  of  which  are  they 
nearly  allied.  Each  on  section  shows  to  the  naked  eye  a 
distinct  yellowish  cortex,  radialh'  striated,  and  a  dark 
brownish  homogeneous  medulla,  the  two  being  clearly  de- 
fined from  each  other.  The  surface  is  invested  by  a  thin 
but  firm  fibrous  capsule  whose  deeper  portion  shows  scat- 
tered bundles  of  smooth  muscular  fibres:  from  it  tough 
fibrous  septa  enter  the  interior  of  the  organ  to  form  the 
cortical  framework,  which  is  limited  internally  b\'  a  con- 
tinuous layer  of  connective  tissue  which  bounds  the  medul- 
la. The  interior  of  the  latter  is  also  pervaded  b}'  a  frame- 
work of  fibrous  tissue. 

The  cortex  is  divided  by  the  difference  in  the  disposition 
of  its  septa  and  in  the  consequent  mode  of  segregation  of 
its  elements  into  three  distinct  zones  which  pass  into  each 
other  without  great  abruptness :  these  are  the  thin  zona 
glomerulosa  just  beneath  the  capsule,  the  zona  fascicu- 
lata  next  within,  which  forms  by  far  the  greater  portion 
of  the  cortex,  and  the  zona  reticularis,  little  if  at  all 
thicker  than  the  outer  zone,  which  lies  next  the  medullary 
sheath  [of  connective  tissue.  The  spaces  of  the  fibrous 
framework  are  occupied  in  the  outer  zone  by  rounded  no- 
dules, in  the  middle  b\'  columnar  masses,  and  in  the  inner 
by  a  network  of  strands  of  closely  packed  polyhedral  cells 
of  moderate  size  whose  protoplasm  shows  numerous  small 
oil  globules  to  which  the  color  of  the  cortex  is  largeh^  due. 

/     A      ^  (?      c 


V.         /  ,/ 


282  PART  II.      HISTOLOGICAL  ANATOMY. 

The  cells  of  the  inner  zone  are  darker  in  color,  frequently 
containing  brownish  pigment :  those  of  the  outer  in  some 
animals  are  occasionalh'  columnar  in  form,  being  disposed 
about  an  ill-defined  lumen  in  the  centre  of  the  nodule. 

The  medulla  contains  within  the  fibrous  stroma  irreg" 
ular  cords  and  masses  of  cells  larger  and  much  more 
loosely  arranged  than  those  of  the  cortex :  they  are  de- 
void of  oil  globules  and  frequentl}^  exhibit  branching  pro- 
cesses. A  rich  plexus  of  non-medullated  nerve  fibres  is 
present,  and  connected  with  great  number  of  ganglion  cells 
either  scattered  or  clustered  in  groups  of  varj'ing  size. 
Numerous  small  ganglia  are  also  found  upon  the  nerves 
just  external  to  the  hilum. 

The  arteries  of  supply  enter  the  surface  of  the  capsule  by 
numerous  small  branches :  wathin,  the  vessels  are  distri- 
buted to  the  cortex  along  its  framework,  the  capillaries 
not  pervading  the  cellular  masses  as  in  the  parathyroids, 
carotid  glands,  and  coccygeal  gland  :  thence  they  pass  to 
the  medulla,  which  contains  a  large  plexus  of  veins  whose 
branches  unite  into  one  at  the  hilum.  The  cortex  is  well 
supplied  with  lymphatics  which  communicate  both  with 
those  of  the  capsule  and  with  those  of  the  medulla. 

In  the  angles  of  the  irregularly  pyramidal  adrenals  of 
the  human  subject  the  cortex  is  folded  upon  itself,  the  me- 
dulla not  extending  into  the  fold ;  the  two  layers  of  the 
zona  reticularis  are,  however,  separated  b^^a  continuation 
of  the  connective  tissue  layer  which  surrounds  the  me- 
dulla. The  distinctness  between  the  cortex  and  the  medulla 
is  associated  with  an  important  difference  in  their  embry- 
onic development.    The  two  arise  independently  of  each 


CHAPTER    XXI.    THE  DUCTLESS  ORGANS.  288 

other,  and,  indeed,  remain  so  throughout  life  in  some  of 
the  fish-like  vertebrates:  the  eortex  arises  as  an  outgrowth 
from  the  peritoneum  in  close  proximity  to  the  mesone- 
phros:  the  medulla  is  derived  from  an  extension  of  the 
adjaeent  sympathetic  chain  of  ganglia  :  from  its  close  con- 
nection with  which  and  from  its  richness  in  nervous  elements 
many  are  inclined  to  regard  the  adrenals  as  essentially 
portions  of  the  nervous  system. 

The  pituitary  body  is  also  known  as  the  hypophysis  x 

cerebri.  It  is  a  double  structure,  consisting  of  an  anterior  j?, 
and  a  posterior  portion :  it  would  be  well  if  the  former 
term  could  be  restricted  to  the  first  of  these  and  the  latter  ,p(^V. 
to  the  other,  since  they  are  essentially  different  alike  in 
structure  and  in  origin.  The  posterior  division  is  in  real- 
ity a  downgrowth  of  the  brain,  as  the  second  term  im- 
plies: it  is  the  rudiment  in  man  and  mammals  of  what  is 
a  distinct  and  important  lobe  of  the  brain  in  the  fish-like 
vertebrates.  The  anterior  is  an  upgrowth  from  the  epi- 
blast  which  lines  the  oral  invagination,  and  is  in  sub- 
stance an  epithelial  body:  it  is  also  a  rudiment  of  what 
was  probabh^  in  the  earlier  vertebrates  or  their  inverte- 
brate ancestors  an  important  glandular  organ;  its  path-  ^^-^■^■ 
ological  relations  indicate  that  it  still  has  a  persistent 
though  as  yet  unknown  function.  The  contact  and  cohe- 
sion of  the  two  bodies  is  confined  to  the  mammals :  in  all 
other  vertebrates  they  remain  distinct. 

The  anterior  lobe  is  larger  than  the  posterior,  and  is  of 
a  darker  reddish  color.  It  consists  of  spheroidal  and  sim- 
ple or  branched[_tubularacini ;  their  closed  cavities  are  lined 


28-4  PART  n.      HISTOLOGICAL  ANATOMY. 

either  by  a  mass  of  polyhedral  ep^itheliqid  cells  which  fill 
the  cavitv,  or  by  a  layer  of  true  epithelial  cells  which  sur- 
round a  distinct  though  sometimes  irregular  lumen  fre- 
quently filled  with  colloid  substance  similar  to  that  found 
in  the  thyroid :  in  some  of  the  larger  tubules  cilia  have 
been  observed  upon  the  cells.  Between  the  vesicles  and 
tubules  is  a  framework  of  connective  tissue  which  sup- 
ports the  numerous  blood  vessels  and  lymphatics  and  is 
continuous  with  the  fibrous  capsule. 

The  posterior  lobe  is  at  first  a  hollow  diverticulum  of 
the  'twixt-brain :  in  man  and  all  mammals  its  cavity  is 
nearly  or  quite  obliterated,  and  it  becomes  a  small  solid 
mass.  The  nervous  tissues  characteristic  of  the  inferior 
lobe  of  the  brain  of  the  fish-like  vertebrates  fail  to  devel- 
ope  in  the  higher  forms,  their  place  being  taken  by  an  in- 
growth of  vessels  and  of  bundles  of  fibrous  tissue:  within 
the  meshes  of  the  latter  are  found  numerous  peculiar  fus- 
iform or  stellate  cells  which  are  frequently  pigmented. 
Where  a  vestige  of  the  cavit\^  remains  it  is  lined  with  col- 
umnar ciliated  cells  similar  to  those  found  lining  the  cavity 
of  the  third  ventricle.  The  relation  of  this  portion  of  the 
pituitary  body  to  the  brain  is  so  close  that  the  whole  is 
frequent^  described  in  connection  wath  that  organ :  but 
the  larger  and  apparently  more  important  anterior  por- 
tion resembles  the  thj^roid  so  much  both  in  structure  and 
in  origin  as  to  justif)^  its  description  among  the  anom- 
olous  ductless  bodies. 

The  pineal  body,  otherwise  known  as  the  epiphysis 
cerebri  has  already  been  spoken  of  in  connection  with  the 


CHAPTER   XXI.      THE    DUCTLESS  GLANDS.  285 

pituitary  body.  Like  the  posterior  portion  of  that  organ, 
it  is  stricth'  a  portion  of  the  brain ;  and  its  structure 
should  be  included  in  a  full  account  of  that  organ.  Like 
its  analogue  first  mentioned,  however,  it  is  in  mammals 
altogether  rudimentary  and  completely  devoid  of  true 
nervous  tissue:  it  has  therefore  no  longer  any  histological 
relation  with  the  nervous  axis  and  may  as  a  matter  of 
convenience  be  described  at  this  time.  It  consists  essen- 
tially of  a  number  of  closed  spheroidal  or  tubular  acini 
lined  or  in  most  cases  wholh'  filled  with  polyhedral 
epithelial  cells,  and  supported  by  dense  interstitial  con- 
nective tissue.  Among  the  epithelial  cells,  as  well  as  on 
the  outer  surface  of  the  body,  are  numerous  gritty  calcare- 
ous particles  known  as  brain-sand:  this  is  also  found  in 
some  other  portions  of  the  brain.  The  pineal  body  in 
some  of  the  lower  vertebrates  contains  true  nervous 
tissue  and  is  connected  with  a  peculiar  median  sense  organ 
sometimes  called  the  parietal  or  pineal  e^-e. 


Attention  was  called  In  the  opening  paragraphs  of  this 
book  to  the  frequent  use  of  the  term  Physiological  Anat- 
omy as  a  s3'nonym  for  Histological  Anatomy.  The  dis- 
tinction between  the  two  is  very  well  illustrated  by  the 
facts  in  the  case  of  the  bodies  described  in  this  chapter. 
It  was  alike  the  hope  and  the  expectation  of  the  earlier 
histologists  and  physiologists  that  the  investigation  of 
the  elemental  structure  of  each  organ  of  the  body  would 
throw  great  light  upon  its  functions.  This  has  proved  to 
be  the  case  in  many  instances,  and  physiology  has  been 


286  PART  II.   HISTOLOGICAL  ANATOMY. 

under  correspondingly  great  obligations  to  histology, 
which  it  has  been  quick  to  acknowledge:  so  ready, indeed, 
that  the  student  new  to  the  subject  is  to  some  extent 
liable  to  loose  sight  of  the  important  fact  that  in  not  a 
few  instances  no  such  happy  result  has  followed  upon 
a  dvances  in  our  knowledge  of  minute  structure.  While, 
for  example,  the  diiference  in  the  form  and  relations  of 
the  cells  lining  the  alveoli  of  serous  and  of  mucous  glands 
respectively  can  be  correlated  with  the  difference  in  the 
character  of  the  fluids  secreted  by  them,  no  one  could 
have  foretold,  given  the  facts  of  structure,  the  associated 
difference  in  function  ;  there  is  still  less  evident  relation  be- 
tween structure  and  function  in  the  case  of  the  cardiac 
and  pjdoric  glands  of  the  stomach ;  and  none  at  all  that 
can  as  yet  be  discerned  between  the  form  and  arrangement 
of  the  hepatic  cells  and  the  secretion  of  bile.  What  is  true 
in  numerous  other  instances  is  eminently  so  in  the  case  of 
the  ductless  bodies :  we  have  in  them  examples  of  quite  com- 
plex organs  whose  histological  anatomy  has  in  each  case 
been  ver}^ carefully  studied;  but  whose  function  is  in  every 
instance,  save  that  of  those  which  consist  chiefly  of 
adenoid  tissue,  almost  if  not  altogether  unknown. 


CHAI'TKH    XXII.      THE  NERVOUS   AXIS.  287 


CHAPTER  XXII. 

THE  CBXTK.\L  NERVOUS  SYSTEM. 


The  ductless  bodies  afford  us  instances  of  organs  exhib- 
iting quite  complex  histological  structure,  concerning 
whose  functions  little  or  nothing  is  known.  A  case  not 
exactly  the  converse  of  this  is  presented  b}'  the  nervous 
axis;  whose  ph3'siological  anatomy  is  exceedingly  com- 
plex, but  whose  histological  structure  is  far  less  so.  For 
example,  experiments  show  that  the  white  matter  of  either 
side  of  the  spinal  cord  is  resolvable  into  a  number  of  more 
or  less  distinct  tracts,  none  of  which  presents  any  single 
structural  feature  or  any  combination  of  features  b}'^  which 
it  can  be  demonstrated :  much  the  same  may  be  said  of 
the  physiological  centres  that  have  been  experimentally 
located  in  the  grey  matter  of  the  cord;  in  no  case  can 
their  positions  be  demonstrated  by  histological  methods. 

Failure  to  distinguish  between  the  provinces  of  ph3''si- 
ological  and  of  histological  anatomy  has  led  many  writ- 
ers upon  the  latter  to  include  in  their  description  of  the 
nervous  axis  many  facts  (in  themselves  of  the  highest  im- 
portance) which  are  demonstrable  only  by  methods  that 
are  essentially  physiological :  to  the  confusion  of  the  stu- 
dent, who  is  frequently  led  to  expect  that  his  sections  will 
show  him  more  than  our  present  histological  methods  at 
least  make  possible;   and  being  disappointed  is  apt  to 


288  PART  n.      HISTOLOGICAL   ANATOMY. 

doubt  the  validity  of  the  statements  which  he  has  read. 
The  present  chapter  will  undertake  no  more  than  a  brief 
description  of  such  structural  features  as  can  with  cer- 
taintv  be  made  out  by  histological  methods;  in  other 
words,  b^'  a  study  of  the  form,  relations,  and  groupings 
of  the  structural  elements  present.  The  consideration  of 
the  different  regions  of  the  brain  wnll  follow  that  of  the 
spinal  cord :  and  both  will  be  preceded  by  a  short  account 
of  the  membranes  which  invest  the  nervous  axis  or  line 
the  cavity  in  which  it  is  contained. 

The  meninges,  or  lining  and  investing  membranes  con- 
nected with  the  nervous  axis,  are  three  in  number:  of 
these  the  outer,  known  as  the  dura  mater,  or  meninx  fi- 
brosa, lines  more  or  less  closely  the  spinal  canal  and  the 
cavity  of  the  skull ;  the  inner,  called  the  pia  mater,  or 
meninx  vasculosa,  closeU^  invests  the  surface  of  the  cord 
and  the  brain;  while  the  third,  the  arachnoid,  or  meninx 
serosa,  situated  between  the  first  and  the  second,  is 
structurally  connected  with  the  latter,  the  two  being  de- 
rived from  the  differentiation  of  a  single  investing  laj'er. 
On  account  of  the  intimate  relation  between  these  two 
meninges  they  are  by  some  anatomists  described  as  one, 
under  the  name  of  the  leptomeninx,  the  outer  membrane 
being  termed  the  pachymeninx :  a  view  which  is  sup- 
ported by  the  comparat  ve  anatomy  of  these  structures 
as  well  as  bj'  their  embryology ;  it  is  also  true,  however, 
that  the  two  meninges  so  designated  are  themselves  form- 
ed from  a  single  mass  of  indifferent  tissue  originally  filling 
the  space  between  the  nervous  axis  and  its  case. 


CHAPTER   XXII.      THE    NERVOUS  AXIS.  289 

Tlie  dura  is  a  thick  and  strong  mass  of  white  fibrous  tis- 
sue, rni.xed  with  a  small  amount  of  elastic  fibres;  the  con- 
stituent bundles  are  dis]3osed  chiefly  in  a  longitudinal  di- 
rection in  the  spinal  portion  of  the  membrane,  those  of  the 
cranial  portion  being  variously  disposed.  In  the  latter  re- 
gion the  dura  is  closely  adherent  to  thecranial  periosteum, 
})articularly  at  the  base  of  the  brain;  dorsally  the  two 
fibrous  layers  are  less  intimatcU'  united;  it  is,  indeed,  cus- 
tomary to  speak  of  the  dura  as  forming  the  intracranial 
periosteum,  and  as  composed  of  two  layers;  but  the  inter- 
pretation here  indicated  is  more  in  accordance  with  the 
facts  of  structure.  The  inner  surface  of  the  cranial  dura  is 
invested  throughout  with  serous  endothelium;  a  similar 
structure  has  been  described  upon  the  outer  surface  in 
places  where  it  is  free  from  the  periosteum.  The  spinal 
dura  is  covered  on  both  sides  with  a  similar  investment. 
In  addition  to  flattened  connective  tissue  corpuscles,  both 
the  cranial  and  the  spinal  dura  contain  numerous  granule 
cells.  The  cranial  dura  is  separated  from  the  cranial 
periosteum  here  and  there  by  the  large  sinuses  which 
receive  the  blood  from  the  veins  of  the  brain,  and  bv 
the  accessory  or  parasinoidal  spaces,  but  is  not  itself 
highly  vascular;  the  same  is  true  of  the  spinal  dura,  be- 
tween which  and  the  periosteal  lining  of  the  spinal  canal 
is  found  the  internal  spinal  plexus.  Ossification  occurs 
normally  in  the  principal  folds  of  the  cranial  dura  (the 
falx  and  the  tentorium)  in  some  mammals. 


The  pia  consists  of  two  more  or  less  clearly  distinguished 
laN'crs,  an  inner  or  investing  layer,  closely  applied  to  the 
subjacent  nervous  mass,  with  whose  connective  tissue 
framework  it  is  directly  continuous ;  and  an  outer  or  vas- 
cular stratum,  rich  in  small  blood  vessels  whose  branches 
enter  the  nervous  tissue  beneath.    The  whole  membrane 


290  PART    II.      HISTOLOGICAL  ANATOMY. 

is  made  up  of  interlacing  bundles  of  white  fibres,  mixed 
with  occasional  elastic  fibres;  in  the  spinal  pia  the  prevail- 
ing direction  of  the  bundles  is  longitudinal.  The  outer 
layer  of  the  pia  is  connected  with  the  arachnoid  by 
numerous  trabeculae  o^  connective  tissue,  which  pass 
across  the  subarachnoid  space;  this  is  quite  extensive 
along  the  spinal  cord,  and  in  some  places  around  the 
brain ;  a  nearly  continuous  series  of  definitely  arranged 
strands  passes  from  the  pia  at  the  sides  of  the  spinal  cord 
across  to  the  inner  surface  of  the  dura,  traversing  the 
arachnoid;  the  groups  of  internally  converging  bundles 
are  found  between  the  spinal  nerves,  with  which  they 
alternate;  the  whole  forms  the  denticulate  ligament  of 
either  side.  Upon  the  free  surface  of  the  pin  and  along  the 
surfaces  of  the  trabeculae  and  the  ligaments  just  de- 
scribed are  found  flattened  connective  tissue  corpuscles, 
endothelioid  in  form  and  position.  In  the  stroma  of  the 
pia  plasma  and  granule  cells  occur,  and  in  some  animals 
great  numbers  of  pigment  cells. 


The  arachnoid  is  composed  of  loosely  interwoven  delicate 
bundles  of  fibres,  the  intervening  meshes  containingnumer- 
ous  flattened  corpuscles:  the  outer  surface  is  invested  with 
a  layer  of  serous  endothelium  ;  it  is  near  to  but  very  spar- 
ingly connected  with  the  dura.  The  inner  surface  corres- 
ponds to  the  outer  surface  of  the  pia,  with  which  it  is 
united  by  the  loose  mesh  work  of  trabeculae  above  de- 
scribed. 

The  relations  of  the  meninges  to  each  other  and  to  the 
adjacent  structures  are  as  yet  but  imperfectly  understood, 
neither  their  structure  nor  their  embryonic  development 
being  as  yet  fully  known.  Such  facts  as  are  known  con- 
cerning the  latter,  and  such  light  as  is  afforded  bj-^  com- 
parative anatomy,  appear    to  justify    the    view    which 


CHAPTER    XXII.      THE  NERVOUS  AXIS.  291 

regards  the  subdural  space  as  the  primary  cleavage  of  the 
mass  of  connective  tissue  which  in  the  embryo  and  in  the 
lowest  vertebrates  lies  between  the  nervous  axis  and  the 
walls  of  the  cerebrosj)inal  canal ;  the  portion  outside  this 
cleft  becoming  the  dura  or  pachymeninx,  and  that  inter- 
nal to  it  tlie  leptomeninx  or  arachno-pial  membrane;  if 
this  view  be  correct,  the  supradural  (or  so-called  "epi- 
dural")  and  the  subarachnoid  spaces  must  be  regarded 
as  secondary.  As  a  modification  of  this  view  it  has  been 
suggested  that  the  arachnoid  is  in  reality  reflected  over 
the  inner  surface  of  the  dura,  thus  forming  a  continuous 
serous  membrane  which  surrounds  a  serous  cavity  for 
which  the  name  of  the  arachnoid  space  (as  a  substitute 
for  the  more  familiar  name  of  subdural  space)  has  been 
proposed ;  and  this  interpretation  has  some  facts  in  its 
support. 

The  spinal  cord,  surrounded  by  its  meninges  and  sup- 
ported by  them,  hangs  freely  in  the  spinal  canal;  through- 
out a  great  part  of  its  extent  it  is  nearly  cylindrical ;  in 
the  cervical  and  lumbar  enlargements  its  transverse 
diameter  is  distinctly  greater  than  the  dorsoventral.  It  is 
divided  into  symmetrical  halves  by  the  so-called  anterior 
and  posterior  fissures;  the  former  of  these,  the  ventral 
fissure  of  comparative  anatomy,  is  a  distinct  furrow  from 
a  fourth  to  a  third  of  the  diameter  of  the  cord  in  depth, 
containing  a  fold  of  the  pia  which  bears  an  important 
relation  to  the  blood  supply  of  the  cord :  the  latter,  or 
dorsal  fissure,  while  deeper  than  the  former,  is  not  a  true 
cleft,  the  right  and  left  masses  being  separated  simph'  bv 
a  stout  median  septum  given  off  from  the  inner  surface 
of  the  pia.  Other  septa  of  a  similar  nature,  but  less  exten- 
sive, are  given  ofif  from  the  pia  at  various  points  along 
the  sides  of  the  cord,  and  contribute  in  some  measure  to 
its  subdivision. 


292  PART  11.      HISTOLOGICAL  ANATOMY, 

When  viewed  in  cross  section  the  cord  appears  to  the 
naked  eye  to  be  made  up  of  two  differently  appearing  sub- 
stances known  from  their  color  as  the  white  and  the 
gray  mitter.  The  latter  is  in  each  half  of  the  cord  almost 
entirely  surrounded  by  the  former,  but  not  uniformly  so  ; 
from  its  central  portion  on  either  side  a  narrow  ridge  pro- 
jects dorsally  (and  slightly  outward)  almost  to  the  sur- 
face of  the  cord ;  from  its  appearance  in  cross  section  it 
is  known  as  the  posterior  or  dorsal  cornu;  in  a  similar 
manner  the  gray  matter  projects  ventrally  and  outward 
to  form  an  anterior  or  ventral  cornu  which  is  much 
thicker  than  the  dorsal,  but  does  not  approach  the  sur- 
face so  nearly;  in  the  thoracic  portion  of  the  cord  a 
slightly  projecting  ridge  extending  directly  outward  be- 
tween the  dorsal  and  ventral  cornu  constitutes  the  so- 
called  lateral  cornu.  The  gray  matter  of  the  two  sides  of 
the  cord  is  continued  inward  in  the  mid-region  through- 
out the  extent  of  the  cord  to  form  the  gray  commissure, 
by  which  the  two  halves  are  connected,  and  in  which  runs 
the  central  canal;  in  consequence  of  the  disposition  of  the 
gray  matter  of  the  cord  its  figure  as  seen  in  cross  section 
is  irregularly  H-shaped. 

The  white  matter  also  exhibits  certain  conspicuous 
subdivisions,  due  in  part  to  the  disposition  of  the  gray 
matter,  in  part  to  the  mode  of  origin  of  the  spinal  nerves. 
The  bundles  of  fibres  which  are  gathered  together  at 
regular  intervals  outside  the  cord  to  form  the  posterior  or 
dorsal  roots  of  the  spinal  nerves  enter  the  cord  close  to  the 
dorsal  cornu  along  a  well-defined  line  on  either  side,  which 
lies  in  a  shallow  groove.  Between  the  dorsal  cornu  of 
either  side  and  the  median  septum,  which  extends  inward 
to  the  gray  commissure,  lies  the  posterior  or  dorsal  col- 
umn of  the  side  in  question  ;  it  is  limited  externally  by  the 
line  of  entrance  of  the  dorsal  roots,  between  which  and 


CHAPTKK    XXII.      THE   NERVOUS  AXIS.  293 

the  dorsal  midline  a  well-marked  septal  process  of  the  pia 
suhdivides  the  column  into  two  important  tracts  to  be 
presently  described.  Between  the  dorsal  and  the  ventral 
cornu  of  each  side  is  situated  the  lateral  column;  anrl  be- 
tween the  ventral  cornu  and  the  ventral  fissure  the  an- 
terior or  ventral  column.  As  has  been  already  stated, 
the  ventral  cornu  docs  not  approach  very  closelv  to  the 
surface,  and  the  fibre-bundles  which  enter  into  the  anterior 
or  ventral  roots  of  the  spinal  nerves  are  given  off  irregu- 
larly over  a  region  of  considerable  width;  there  is  there- 
fore no  clearlv  defined  boundary  either  externally  or 
internally  between  the  lateral  and  the  ventral  columns ; 
the  whole  region  from  the  dorsal  cornu  to  the  ventral  fis- 
sure is  therefore  sometimes  spoken  of  as  the  antero- 
lateral or  lateroventral  column.  The  ventral  columns  of 
the  two  sides  are  connected  with  each  other  dorsad  of  the 
ventral  fissure  by  a  narrow  layer  of  white  matter  called 
the  white  commissure. 

The  ajjpearances  thus  far  described  can  all  be  readily 
seen,  most  of  them  with  the  naked  eye;  the  histological 
structure  of  which  they  are  the  expression,  in  some  re- 
spects exceedingly  simple,  is  in  others  of  such  complexity 
that  it  has  hitherto  taxed  the  resources  of  histological 
technique  to  the  utmost.  It  has  long  been  known  that  the 
gray  matter  contains  numerous  multipolar  nerve  corpuscles 
of  varying  size  and  of  a  distinct  but  not  sharply  defined 
arrangement;  their  branching  processes;  non-medullated 
nerve  fibres;  and  numbers  of  the  neuroglia  cells  mentioned 
in  a  previous  chapter;  that  the  central  canal  is  lined  with 
a  layer  of  columnar  cells  which  are  sometimes  ciliated ; 
that  the  white  matter  is  composed  chiefly  of  medullated 
nerve  fibres,  mingled  with  which  are  numerous  neuroglia 
cells  like  those  of  the  gray  matter:  that  the  pial  septa 
already  mentioned  subdivide  in  the  interior  of  the  cord  to 


294  PART    11.       HISTOLOGICAL  ANATOMY, 

form  a  framework  along  which  are  distributed  the  inter- 
nal vessels  of  the  cord,  the  general  disposition  of  which 
can  readih'^  be  determined  In  brief,  the  cord  is  known  to 
consist  of  multipolar  corpuscles  and  nerve  fibres  sup- 
ported largely  by  neuroglia  cells  and  provided  with  a 
definite  blood-supply.  The  complexity  of  the  cord  depends 
not  upon  the  number  of  kinds  of  tissue  elements  present, 
no  other  active  organ  in  the  body  of  equal  size  and  im- 
portance containing  so  few;  but  upon  the  arrangement  of 
the  corpuscles  and  fibres,  and  particularly  upon  the  rela- 
tions existing  between  them ;  and  it  is  only  recently  that 
the  latter  has  been  at  all  satisfactorily  determined  by 
means  of  technical  processes  of  great  efficiency  and  corres- 
ponding delicacy.  It  will  be  profitable  to  consider  first 
the  more  conspicuous  and  therefore  longer  known  feat- 
ures of  the  histological  anatomy  of  the  cord,  and  subse- 
quently the  minuter  details  that  have  more  recently  been 
discovered. 

Beginning  with  the  gray  matter,  the  most  conspicuous 
and  in  every  sense  the  most  important  elements  are  the 
multipolar  corpuscles  scattered  throughout  it.  While 
they  at  first  seem  to  be  quite  irregularly  distributed,  the 
examination  of  a  number  of  successive  sections  from  the 
same  region  of  the  cord,  and  theircomparison  with  similar 
series  from  other  regions  will  demonstrate  that  the  corpus- 
cles are  arranged  in  groups  extending  along  the  length  of 
thecord  throughout  the  whole  or  definite  portions  of  its  ex- 
tent with  sufficient  regularitv  to  warrant  their  design  a- 
tion  as  distinct  ganglionic  columns.  Of  these  the  most 
readily  distinguishable  on  account  alike  of  its  extent  and 
the  size  of  its  constituent  corpuscles  is  the  column  of  the 
ventral  cornu,  or,  from  the  know^n  distribution  of  the 
axis-cylinder  processes  of  most  of  its  corpuscles  to  the  ven- 
tral roots  of  the  spinal  nerves,  the  motor-corpuscle  col- 


CHAPTER    XXII.      THE   NERVOUS  AXIS.  295 

umn;  this  mav  a^aiii  be  more  or  less  distinctly  divided  in 
some  portions  of  the  cord  into  a  mesial  tract,  situated 
adjacent  to  the  ventral  fissure  (  which  probably  supplies 
motor  fibres  to  the  nerves  of  the  dorsal  muscles  ol  the 
trunk);  a  ventrolateral  tract,  contiguous  to  the  mesial 
and  extending  to  the  outer  side  of  the  cornu,  (which 
probal)lv  supplies  fibres  to  the  nerves  of  the  lateral  and 
ventral  muscles  of  the  trunk) ;  these  two  tracts,  which  are 
confluent  in  the  thoracic  portion  of  the  cord,  are  quite  con- 
st'itit  throughout  its  whole  extent;  and  a  third,  the. dor- 
solateral tract,  which  is  situated,  as  its  name  implies,  in 
the  outer  side  of  the  cornu  and  dorsad  of  the  second  above 
mentioned  ;  it  is  present  chieflv  in  the  cervical  and  lumbar 
enlargements  (  and  probablv  supplies  fiiires  to  the  brachial 
and  sacral  plexuses).  Still  another  group,  more  centrally 
located,  can  be  distinguished  in  some  sections;  but  it  is 
much  less  constant  than  those  above  described  The  cor- 
puscles of  the  ventral  cornu  are  the  largest  found  in  the 
cord,  their  diameters  ranging  in  most  instances  from 
sixty-five  to  one  hundred  and  thirty  micra. 

At  the  base  of  the  ventral  cornu  and  near  the  gray  com- 
missure is  a  group  of  much  smaller  corpuscles,  rarely  ex- 
ceeding thirty  micra  in  diameter,  the  medlocentral  tract; 
it  is  well  developed  in  the  thoracic  portions  of  the  cord, 
but  much  less  so  in  the  cervical  and  the  lumbar.  At  the 
base  of  the  so-called  lateral  cornu,  also  in  the  thoracic  re- 
gion, there  is  a  group  of  corpuscles  of  similar  size,  the 
mediolateral  tract,  or,  as  it  is  called,  the  column  of  the 
lateral  cornu. 

The  dorsal  cornu  exhibits  at  its  juncture  with  the  gray 
commissure  a  well-marked  column  which  extends  from  the 
level  of  the  seventh  cervical  to  that  of  the  third  lumbar 
nerve  ;  throughout  the  thoracic  region  it  is  quite  conspic- 
uous, being  roughly  cylindrical  in  form  and  quite  sharply 
defined  by  the  arrangement  of  the  adjacent  fibres;  it  was 


296  PART  II.      HISTOLOGICAL  ANATOMY. 

the  first  column  ofcorpuscles  to  be  clearl}' recognized,  and  is 
known  from  its  discoverer  as  Clarke's  column.  Stilling 
proposed  for  it  the  name  of  the  dorsal  ( thoracic  j  nucleus, 
and  for  similarly  placed  aggregations  situated  respectively 
at  the  level  of  the  third  and  fourth  cervical  nerves  and  the 
second  and  third  sacral  the  names  of  the  cervical  and  the 
sacral  nucleus.  The  corpuscles  of  Clarke's  column  are 
next  to  those  of  the  anterior  cornu  in  size,  being  from 
thirty  to  ninety  micra  in  diameter.  Small  corpuscles, 
from  twenty  to  thirty  micra  in  diameter,  are  scattered 
through  the  rest  of  the  cornu  with  little  definite  arrange- 
ment; those  close  to  the  mesial  margin  are  in  some  cases 
quite  distinctly  elongated  parallel  thereto  and  are  known 
as  marginal  corpuscles;  near  the  middle  of  the  cornu  the 
gray  matter  is  broken  up  along  the  mesial  side  and 
to  some  extent  along  the  lateral  side  also  by  bundles 
of  fibres;  the  reticular  formations  thus  produced 
have  associated  with  them  irregularly  defined  groups  of 
corpuscles.  The  posterior  cornu  is  capped  dorsally  by  a 
trani*lucent  mass,  rich  in  glia  cells,  known  as  the  gelati- 
nous substance  of  Rolando ;  it  contains  numerous  small 
rounded  elements,  about  fifteen  micra  in  diameter,  gener- 
ally regarded  as  nerve  corpuscles,  as  well  as  other  larger 
elements,  unquestionably  nervous,  some  of  which  are  mar- 
ginal in  form  and  position.  Mention  may  also  here  be 
made  of  the  fact  that  various  observers  have  described 
outlying  corpuscles  scattered  among  the  fibres  of  the 
white  columns,  both  dorsal  and  lateroventral. 

While  the  corpuscles  are  the  most  conspicuous  elements 
of  the  gray  matter,  they  form  but  a  small  portion  thereof. 
By  far  the  larger  portion  consists  of  a  densely  felted  mass 
of  fibres  of  various  kinds ;  including  small  medullated 
fibres,  nonmedullated  fibres,  axis-cylinder  processes  and 
their  branches,  and  the  dendritic  subdivisions  of  the 
protoplasmic    processes  of  the  corpuscles.     These    were 


CHAPTER   XXII.      THE  NERVOUS  AXIS.  297 

formerly  suj)posc{l  to  be  continuous  .'ind  to  form  the  so- 
called  reticulum  of  the  gray  matter:  but  the  evidence  of 
recent  investi^^ations  by  methods  giving  results  of  great 
delicacy  leads  to  the  belief  that  no  such  continuity  exists; 
what  is  now  generally  believed  to  be  the  true  relation  of 
the  corpuscles  and  the  fibres  of  the  gray  matter  will  be  in- 
dicated later.  To  the  interlacing  fibres  and  fibrillae 
already  mentioned  are  added  the  numerous  delicate  fila- 
ments proceeding  from  the  glia  cells  which  with  the  con- 
nective tissue  elements  present  make  up  what  is  some- 
times called  the  spongy  substance  of  the  gray  matter. 
About  the  central  canal  there  lies  a  translucent  layer  simi- 
lar to  that  which  caps  the  dorsal  cornua,  the  central 
gelatinous  substance. 

The  white  matter  of  the  cord  consists,  as  has  already 
been  said,  chiefly  of  medullated  nerve  fibres;  these  vary 
greatly  in  size,  the  smallest  being  but  one  or  two  micra 
in  diameter,  while  the  largest  may  have  a  diameter  of  over 
twenty-five  micra.  The  great  majority  of  these  fibres  are 
longitudinally  disposed,  the  most  conspicuous  exception 
to  this  being  found  in  the  white  commissure,  through 
which  fibres  pass  at  greatly  varying  degrees  of  obliquit}'. 
Between  the  medullated  fibres  are  numerous  neuroglia 
cells  so  disposed  as  to  form  an  interstitial  framework  be- 
tween the  subdivisions  of  the  pial  septa.  Along  the  sur- 
face of  the  septa  the  neuroglia  cells  are  gathered  in  great 
numbers,  forming  a  definite  investment  which  is  con- 
tinuous outwardl}'^  with  a  well  defined  layer  of  consider- 
able thickness  which  lines  the  inner  surface  of  the  pia. 

While  it  is  possible  to  recognize  readily  that  certain 
regions  of  the  cord,  presently  to  be  indicated,  contain 
chiefly  cither  larger  or  smaller  medullated  fibres,  through 
most  portions  the  fibres  vary  so  much  and  so  irregularl}^ 
in  size  that  no  such  division  into  tracts  can  be  made  as  in 


298  PART  It.     HISTOLOGICAL  A.NATOMV. 

the  gray  matter,  based  on  the  size  and  arrangement  of  the 
fibres;  the  only  conspicuous  visible  feature  on  which  a  sub- 
division can  be  based  is  the  presence  of  the  important 
secondary  septum  already  mentioned  as  situated  between 
the  median  septum  and  the  dorsal  cornu  of  either  side; 
this  passes  obliquely  toward  the  ventral  edge  of  the 
median  septum,  thus  dividing  the  dorsal  column  into  two 
well  defined  portions,  long  known  respectively  as  the 
column  of  GoU  (next  to  the  median  septum)  and  the  col- 
umn of  Burdach  (next  to  the  dorsal  cornu).  The 
remainder  of  the  cord  has  been  more  or  less  accurately 
subdivided  into  tracts  by  other  methods;  while  these 
tracts  cannot  be  distinguished  by  any  means  now  in  our 
possession  in  sections  of  the  normal  adult  cord,  some 
knowledge  of  them  is  necessary  in  order  to  understand 
the  meaning  of  some  of  the  finer  details  of  structure  devel- 
oped by  recent  histological  researches  in  both  the  white 
and  gray  matter:  a  brief  account  of  them  and  of  the 
methods  by  which  they  have  been  determined  will  there- 
fore be  given. 

We  owe  chiefly  to  Flechsig  the  discovery  that  the  medul- 
lated  nerve  fibres  of  the  different  tracts  of  the  cord  attain 
their  full  development  at  different  periods  of  embryonic 
and  even  of  infantile  life ;  the  difference  in  question  being 
apparently  correlated  with  similar  differences  in  thecalling 
into  activity  of  different  powers  and  faculties  of  the  ner- 
vous system :  the  study  of  the  spinal  cords  of  embryos  at 
various  stages  of  advancement  thus  revealing  significant 
differences  in  structure  not  discernible  in  the  cord  of  the 
adult.  To  Waller  we  are  indebted  for  pointing  out  that 
an  axis-cylinder  when  severed  from  the  corpuscle  of  which 
it  is  a  process  rapidly  undergoes  degeneration :  it  follows 
from  this  that  in  cases  of  lesion  of  the  cord,  either  patho- 
logical or  experimental,  not  only  single  fibres  but  also 
tracts  composed  of  fibres  of  a  similar  character  will  un- 


CHAPTER  XXII.      TIIK   \ERVOUS  AXIS.  209 

dergo  degeneration  in  a  manner  definitely  related  to  the 
place  of  lesion.  Without  entering  into  the  physiological 
significance  of  the  changes  involved,  it  may  be  said  that 
degeneration  on  the  side  of  the  lesion  toward  the  brain  is 
called  ascending;  and  that  on  the  opposite  side  of  the 
lesion  descending ;  and  that  the  same  terms  are  applied 
to  the  tracts  in  which  the  changes  in  question  occur.  The 
method  of  Flechsig  and  that  based  on  the  Wallerian  law 
of  degeneration  give  us  results  which  coincide  to  such  an 
extent  as  to  determine  quite  clearly  the  limit  of  some  of 
the  tracts  about  to  be  mentioned ;  the  evidence  in  favor  of 
the  existence  of  others,  while  generally  regarded  as  satis- 
factory, is  by  no  means  as  conclusive. 

None  of  the  tracts  which  are  thus  shown  to  exist  in  the 
white  matter  of  the  spinal  cord  are  more  clearly  definable 
than  those  which  pass  from  the  pyramids  of  the  medulla 
oblongata  into  the  ventral  and  lateral  columns  of  the 
cord.  The  fibres  of  the  pyramids  decussating  while  still  in 
the  medulla,  the  great  majority  of  them  enter  a  large  com- 
pact bundle  whose  cross  section  is  an  irregularly  triangular 
area  lying  (in  the  human  spinal  cord)  between  the  dorsal 
cornu,  from  which  it  is  separated  by  a  thin  layer  of  fibres, 
and  the  lateral  surface  of  the  cord,  from  which  it  is  also 
separated  by  a  layer  of  fibres  save  in  the  lumbar  region, 
where  it  extends  to  the  surface ;  it  is  known  as  the  lateral 
(or  the  crossed)  pyramidal  tract.  In  man  the  fibres  of  the 
pyramid  do  not,  as  a  rule,  all  decussate  in  the  medulla,  a 
small  tract  passing  down  as  a  thin  la\'er  on  the  portion 
of  the  surface  of  the  ventral  column  which  lies  within  the 
ventral  fissure  on  the  same  side  as  the  pyramid  from  which 
it  proceeds;  this  tract  is  therefore  called  the  anterior  (or 
the  direct)  pyramidal  tract ;  it  does  not  extend  beyond 
the  middle  of  the  thoracic  region  of  the  cord.  It  is  proba- 
ble that  decussation  goes  on  between  the  right  and  left 
tracts  throughout  their  whole  course,  the  fibres  passing 


300  PART  II.      HISTOLOGICAL  ANATOMY. 

throuo^h  the  white  commissure,  instead  of  occurring  all  at 
once  in  the  medulla:  in  some  mammals  the  pyramidal 
decussation  is  complete,  and  there  is  in  consequence  no 
direct  pyra.nidal  tract:  this  is  sometimes  the  case  in  the 
human  subject  The  existence  of  both  pyramidal  tracts 
was  demonstrated  by  Tuerck,  though  his  name  is  usually 
associated  with  the  smaller  and  less  constant  of  the  two, 
which  is  CO  nmonly  designated  the  column  of  Tuerck. 

Far  less  clearly  defined  and  less  constant  in  position  is 
another  descending  tract,  the  ventrolateral,  or  anterior 
marginal  bundle  of  Loewenthal:  it  consists  of  a  thin 
layer  of  fibres  situated  upon  or  near  the  surface  of  the 
ventral  and  a  good  portion  of  the  lateral  columns :  its 
fibres  proceed  fro  n  the  cerebellar  cortex  ot  the  same  side. 

In  close  contact  with  the  ventrolateral  descending 
cerebellar  tract,  the  fibres  of  the  two  mingling  to  a  greater 
or  less  extent,  lies  the  ventrolateral  ascending  cerebellar 
tract,  or  anterolateral  ascending^  tract  of  Gowers:  like 
its  companion,  it  is  throughout  the  larger  part  of  its 
course  a  thin  layer  of  fibres,  which  is  situated  between  the 
tract  just  mentioned  and  the  surface  of  the  cord:  it  is 
thickest  in  its  most  dorsal  portion,  and  is  limited  in  that 
direction  by  the  crossed  pyramidal  tract. 

Dorsad  of  the  tract  of  Gowers  and  external  to  the 
crossed  pyramidal  tract  is  the  dorsolateral  ascending 
cerebellar  tract,  or  the  direct  cerebellar  tract  of  Flechslg. 
It  begins  in  the  lower  part  of  the  thoracic  portion  of  the 
cord  (below  which  level  the  crossed  pyramidal  tract  comes 
to  the  surface  of  the  cord)  and  extends  from  there  up- 
wards, passing  through  the  restiform  body  in  the  medulla 
oblongata  to  reach  the  middle  lobe  of  the  cerebellum.  Like 
the  pyramidal  tracts  and  unlike  those  just  described,  the 
tract  of  Flechsig  is  very  clearly  defined.  Between  its  dor- 
sal border  and  the  line  of  entrance  of  the  dorsal  roots  of 
the  spinal  nerves,  limited  externally  by  the  surface  of  the 
cord,  is  a  narrow  tract,  the  marginal  zone  of  Lissauer. 


CHAPTER    XXII.      THE   NERVOUS  AXIS.  301 

Attempts  have  been  made  to  farther  subdivide  the  white 
matter  of  the  ventral  and  lateral  columns;  but  the  results 
on  which  these  attempts  arc  based  arc  thus  far  so  conflicting 
as  to  render  theconclusions  drawn  therefrom  cpiite  doubt- 
ful: for  the  present  it  is  best  to  include  the  whole  of  the 
territory  enclosing  the  ventral  cornu  and  perforated  by 
the  fibre-bundles  of  the  ventral  roots  of  the  spinal  nerves 
(excepting,  ol  course,  the  tracts  already  designated)  under 
the  title  of  the  ventro-lateral  root  zone. 

The  columns  of  GoU  and  ol  Burdach,  situated  in  the 
dorsal  region  of  the  white  matter,  have  already  been  de- 
scribed as  limited  structurally:  they  may  be  otherwise 
distinguished,  according  to  their  function,  as  the  dorso- 
median  and  the  dorso- lateral  ascending  tracts.  Im- 
bedded within  the  latter  may  be  detected  a  small  bundle 
of  fibres  with  descending  degeneration,  known  Irom  its 
outline  when  seen  in  cross  section  as  the  comma. 

Wehaveseen  that  some  of  the  tracts  above  described,  both 
ascending  and  descending,  are  in  direct  relation  with  the 
brain  :  others  are  doubtless  composed  entirely'  or  in  large 
measure  of  fibres  that  begin  and  end  in  the  cord  itself: 
others,  and  pcirticularly  those  of  the  dorsal  tracts,  are  in 
direct  relation  with  the  spinal  nerves;  and  these  latter 
organs  are  in  such  close  connection  with  the  cord  as  to 
merit  mention  in  this  connection. 

Each  spinal  nerve  possesses,  as  is  well  known,  a  dorsal 
and  a  ventral  root.  The  latter  consists  of  efferent  fibres 
chiefly  if  not  solely  motor  in  function,  which  arise  from  the 
axis-cylinder  processes  of  the  corpuscles  of  the  ventral  cornu, 
as  has  been  stated,  and  pass  almost  directly  out  of  thecord  : 
they  therefore  make  no  important  contributions  to  its  col- 
umns. The  dorsal  root,  in  addition  to  its  ganglion,  which 
will  be  farther  discussed  in  a  subsequent  paragraph,  con- 
tains a  few  fibres  probably  motor  in  function :  it  consists 
chiefly  of  efferent  or  so  called  sensory  fibres  which  are  some- 


302  PART  11.      HISTOLOGICAL  ANATOMY. 

what  definitely  divided  into  two  groups,  a  mesial  and  a  lat- 
eral, in  each  bundle;  they  penetrate  the  surface  of  the  cord 
more  or  less  obliquely  and  then  bifurcate,  givmg  rise  to  as- 
cending and  descending  branches,  the  disposition  of  whose 
terminal  and  collateral  arborizations  will  be  described  later. 
Recent  researches  by  Cajal  indicate  that  the  fibres  of  the 
lateral  group,  which  are  slender,  have  their  bifurcation  in 
the  marginal  zone  of  Lissauer  and  the  adjacent  part  of 
the  lateral  column;  their  collaterals  are  few  and  delicate 
and  end  in  the  dorsal  cornu  :  the  fibres  of  the  mesial  group, 
which  are  stouter,  reach  the  columns  of  Goll  and  Burdach, 
and  there  bifurcate:  their  collaterals  form  by  far  the  larger 
portion  of  those  subsequently  to  be  described  as  derived 
from  the  dorsial  column,  and  in  particular  those  which 
form  the  channels  whereby  those  impulses  are  transmitted 
which  are  involved  in  simple  reflex  movements. 

The  white  matter  of  the  cord,  therefore,  consists  of  med- 
ullated  nerve  fibres  which  may  be  divided  according  to 
their  origin  and  destination  into  three  groups,  the  mem- 
bers of  which  are  not  structurally  distinguishable:  those 
which  pass  from  the  cord  to  end  in  the  brain;  those,  com- 
missural in  character,  which  pass  from  one  portion  of  the 
cord  to  end  in  another;  and  those  which  pass  to  the  cord 
from  the  brain  or  from  the  spinal  nerves.  From  the  de- 
scription of  the  nervous  elements  given  in  a  preceding 
chapter  it  will  be  evident  that  each  of  these  fibres  consists 
essentially  of  the  axis-cylinder  process  of  a  corpuscle,  and 
ends  in  an  arborization,  giving  off  along  its  course  one  or 
more  collaterals.  It  is  probable,  but  not  certain,  that 
these  collaterals  have  their  terminal  arborizations  in  re- 
gions of  the  gray  matter  of  the  cord  homologous  with 
that  in  which  the  terminal  arborization  of  the  fibre  itself 
is  situated. 

Leaving  out  of  consideration  for  the  present  the  fibres 


CttAPTEk  XXll.     THE  NERVOUS  AXIS.  303 

which  pass  from  the  cord  to  the  l)rain,  it  may  be  said  that 
the  terminal  arborizations  of  fibres,  whether  collateral  or 
principal^  which  enter  the  gray  matter  of  the  cord  are  in 
close  contiguitv  or  actual  contact  with  the  bodies  or  the 
dendritic  processes  of  the  corpuscles  of  the  gray  matter: 
the  latter  play  the  part  of  conductors  (and  not  a  nutritive 
role  merely),  and  connection  is  thusestablished  asefficiently 
ns  by  the  continuity  of  substance  once  supposed  to  exist. 
An  impulse  thus  transmitted  calls  forth  the  activity  of  the 
corpuscle  in  question,  resulting  in  a  discharge  along  its 
axis-cylinder  process  which  undergoes  a  similar  distribu- 
tion. 

Regarding  the  final  arborization  of  a  fibre  as  essentially 
similar  to  thoseof  the  collaterals,  and  therefore  to  beclassi' 
fied  with  them,  we  shall  make  the  first  step  toward  a  con* 
ception  of  the  physiological  anatomy  of  the  cord  by  an 
enquiry  into  the  distribution  of  these  structures  as  they 
leave  the  various  regions  of  the  white  matter.  The  follow- 
ing account  thereof,  as  well  as  that  of  the  corpuscles  of  the 
gray  matter  to  be  subsequently  given,  is  taken  almost 
wholly  from  Cajal. 

Collaterals  of  the  ventral  (anterior)  column.  These  are 
larger  than  those  from  any  other  portion  of  the  cord : 
springing  from  the  large  axis  cylinders  which  compose  this 
column  they  pass  dorsad  in  irregular  groups  to  be  distrib- 
uted within  the  ventral  cornu  and  particularly  about  the 
motor  corpuscles.  Some  bundles  pass  to  the  mid-plane  of 
the  cord  and  are  distributed  in  the  ventral  cornu  of  the 
opposite  side,  constituting  the  ventral  (or  anterior)  com- 
missure  of  collaterals,  situated  largely  dorsad  of  the  ven- 
tral commissure  formed  of  axis  cylinders. 

Collaterals  of  the  lateral  column.  These  pass  inward 
to  be  distributed  chiefly  in  the  central  region  of  the  gray 


304  PART  II.     HISTOLOGICAL  ANATOMY. 

matter  of  the  same  side:  some,  however,  pass  to  the  mid- 
plane,  dorsad  of  the  central  canal,  where  thev  form  a  por- 
tion of  the  dorsal  (posterior)  or  gray  commissure:  they 
are  divided  in  it  into  two  bundles,  a  ventral  and  a  ne- 
dian,  and  are  distributed  in  the  central  and  to  some  extent 
the  dorsal  region  of  the  opposite  side 

Collaterals  of  the  dorsal  (posterior )  column.  The  tracts 
of  Goll  and  of  Burdach,and  the  marginal  zone  of  Lissauer, 
from  which  these  collaterals  are  chiefly  derived,  are  formed 
in  great  part  of  the  continuations  of  the  fibres  of  the  pos- 
terior roots  of  the  spinal  nerves.  Four  groups  of  collat- 
erals may  be  distinguished. 

Sensitivo-motor  (or  reflexo-motor)  collaterals:  these 
arise  not  onh'  from  the  continuations  of  the  fibres  of  the 
posterior  root  but  also  from  the  fibres  themselves  before 
their  bifurcation;  passing  across  the  gray  matter,  they 
terminate  in  the  ventral  cornu  of  the  same  side. 

Dorsal  cornu  collaterals :  these,  like  the  preceding,  are 
very  numerous:  they  traverse  the  substance  of  Rolando 
in  groups  to  form  immediately  ventrad  thereof  and  through- 
out the  substance  of  the  dorsal  cornu  a  dense  network 
composed  of  the  intercrossing  of  their  terminal  arboriza- 
tions. 

Clarke's  column  collaterals:  small  bundles  pass  ven- 
.  trally  from  the  tract  of  Goll  to  terminate  in  the  column  of 
Clarke  of  the  same  side,  there  forming  a  thick  network 
about  the  corpuscles  of  the  column. 

Commissural  Collaterals:  arising  chiefly  in  the  tract  of 
Goll  numerous  small  bundles  pass  to  the  most  dorsal  por- 
tion of  the  gray  commissure,  which  they  traverse,  to  be 
distributed  in  the  dorsal  cornu  of  the  opposite  side:  thus 
forming  the  dorsal  bundle  of  the  commissure. 

Thus,  it  will  be  seen,  each  white  column  gives  off  two 


CIIAPTKR    XXII.      THE    NKRVOUS  AXIS.  305 

kinds  of  coUateriils:  those  which  furnish  their  terminal 
arborizations  to  the  gray  matter  of  the  same  side,  and 
those,  commissural  in  chiiracter,  which  are  destined  to 
ramifv  in  the  gray  matter  of  the  opposite  side.  In  either 
case  their  relations  are  eventually  directly  with  the  cor- 
])uscles  of  the  gray  matter,  whose  disposition  may  now 
be  further  considered.  E.Kcepting  in  the  substance  of  Ro- 
lando, where  some  elements  of  special  form  ai*e  found,  the 
corpuscles  of  the  dorsal,  central,  and  ventral  regions  diflfer 
but  little,  save  in  size.  The  only  important  distinction  to 
be  noted  in  them  pertains  to  the  final  disposition  of  their 
axis-cylinder  processes:  on  this  basis  five  groups  or  kinds 
of  corpuscles  may  be  distinguished  :  of  these  the  first  four 
send,  as  will  be  seen,  their  axis-cylinder  processes  out  of 
the  gray  matter  into  the  white,  there  to  become  medul 
lated :  they  may  be  therefore  distinguished  as  corpuscles 
with  long  axis-cylinder  processes  (corpuscles  of  the  first 
class);  while  the  axis-cvlinder  processes  of  the  fifth  end  in 
the  gray  matter  not  far  from  their  point  of  origin :  they 
are  therefore  corpuscles  with  short  axis-cjdinder  processes 
(corpuscles  of  the  second  class,  corpuscles  of  Golgi).  The 
five  kinds  of  corpuscles  are  as  follows: 

Radical  corpuscles,  or  corpuscles  directly  related  to  the 
roots  of  the  spinal  nerves.  These  are  the  motor  corpuscles 
of  the  ventral  cornua,  and  comprise  the  largest  corpuscles 
of  the  cord :  their  axis-cylinder  processes  are  thick  and 
devoid  of  collaterals  and  pass  in  most  cases  directly 
through  the  lateroventral  column  to  enter  the  ventral 
roots  of  the  spinal  nerves.  From  a  few^  of  the  corpuscles 
the  axis-cylinder  processes  traverse  the  gray  matter  to 
leave  the  cord  by  the  dorsal  root :  they  pass  through  the 
spinal  ganglia,  however,  without  entering  into  relation 
with  their  corpuscles,  and  must  be  regarded  as  in  all  prob- 
ability motor  in  function. 


Q'()6  PART  n.      HISTbLOGICAL  ANATOMY? 

■  The  dendrites  of  these  corpuscles  are  stout,  lotig/aria 
ver}'  much  branched.  They  ma}'  be  distinguished  as  ven- 
tro-external,  dorsal,  and  internal  (mesial);  the  latter 
branch  dichotomously  in  the  Vicinity  of  the  ventral  (ante- 
rior) commissure ;  some  of  the  branches  pass  the  niid-plane 
and  enter  the  ventral  cornu  of  the  Opposite- side,  intercross- 
ing with  corresponding  processes  therefrom  and  forming 
the  protoplasmic  commissure  of  Caijal.  The  ventro-ex- 
ternal  processes  terminate  in  the  lateroventral  column, 
and  the  posterior  in  different  regions  of  the  ventral  cornu. 

Commissural  Corpuscles:  these  are  smaller  than  thosie 
just  described,  a:nd  provided  vC^ith  fewer  arid  shorter  den- 
drites. Golgi  demonstrated  their  presence  in  all  of  the 
regions  of  the  gray  matter,  and  that  the  axis-cylinder  pro- 
cesses pass  to  the  mid-plane  which  they  crOss  ventrally  (Iti 
the  white  commissure)  to- be  continued  to  the  ventrolat^- 
eral  column  of  the  Opposite  side.  Cajal  has  shown  that 
thfey  there  under'g'o  not  a  sifi'gle  continuation',  btit  a  T- 
division:  this  indicates  that'the  conitnissural  axis-cylinder 
process,  on  reaching  the  white  inatter  of  the  opposite  side; 
divides  into  an  ascending  and  a  descending  fibre  of  the 
column. 

Columnar  corpuscles :  we  may  thus  designate  the  nutn- 
erous  medium-sized  corpuscles  scattered  throughout  the 
whole  of  the  gray  matter,  of  which  the  axis-cylinder  pro- 
cesses enter  into  vertical 'fibres  of  their  own  side.'  The 
greater  number  of  the  corpuscles  of  this  kind  which  occur 
in  the  ventral  cornii  send  their  processes  to  the  lateroven- 
tral column :  those  which  are  sittJated  in  the  dorsal  cornu 
direct  them  toward  the  most  dorsal  portion  of  the  lateral 
column  in  manycases,  though  some  of  the  corpuscles  found 
in  the  substance  of  Rolando  and  the  internal  portion  of 
the  dorsal  cornu  send  their  processes  to  the  dorsal  column. 


CHAPTKR    XXII.      THE   NHRYODS  AXIS;  307 

A?  rc«;ar(ls  tht'columii  ol  Clarke,  two  kinds  of  corpuscles 
cati  be  demonslrated :  conimissiiral  corpuscles,  whose 
processes  enter  the  ventral  commissure  ;  and  columnar  cor- 
puscles, whose  processes  pass  to  the  lateral  column  to  be- 
come continuous  with  the  fibres  of  the  cerebellar  tract. 
This  continuation  takes  place  by  two  methods;  by  the 
formation  of  a  l)end,  which  furnishes  a  sinj^le  conductor, 
ascendiuLj  or  descending;  and  by  a  T-division,  which  forms 
two  conductors  or  vertical  branches,  one  ascending  and 
the  other  descending. 

Pluricolumnar  corpuscles :  Elements. are  so  termed  by 
Cajal  of  which  the  axis-cylinder  process  is  divided  while 
still  in  the  gray  matter  into  two  or  more  portions  which 
enter  into  as  many  nerve  fibres  belonging  to  different  col- 
umns :  thus,  for  example,  an  element  of  this  kind  may  send 
one  fibre  to  the  ventral  column  of  its  own  side  and  an- 
other to  that  of  the  opposite  side;  in  another  the  process 
may  divide  into  a  fibre  for  the  dorsal  column  and  another 
for  the  lateral  or  ventral,  etc. 

Van  Gehuchten  has  proposed  for  the  corpuscles  here 
designated  commissural,  columnar,  and  pluricolumnar 
the  names  of  heteromeral,  tautomeral,  and  hecateroraeral 
corpuscles  respectively. 

Short  process  corpuscles:  these,  which  are  found  chiefly 
in  the  dorsal  portion  of  the  gray  matter,  have  slender  and 
flexuous  axis-cylinder  processes  which  speedily  end  in  arbor- 
izations situated  in  proximit\'  to  Other  and  adjacent  cor- 
puscles. 

The  substance  of  Rolando  merits  special  mention  on  ac- 
count of  the  peculiarities  of  some  of  the  corpuscles  con- 
tained therein.  The  latter  belong  chiefly  to  the  columnar 
and  short  process  types,  with  some  pluricolumnar  cor- 


308  PART  II.      HISTOLOGICAL  ANATOMY. 

puscles;  whilst  commissural  corpuscles  are  not  known 
with  certaint}'  to  occur.  Those  characteristic  of  the  re- 
gion are  of  three  principal  forms  disposed  in  as  manv  con- 
centric zones,  passing  from  without  inward,  as  follows. 

The  marginal  corpuscles  are  large  fu:!iform  or  flat- 
tened elements  situated  between  the  substance  of  Rolando 
and  the  dorsal  column  of  white  matter,  thus  forming  a 
discontinuous  layer.  The  dendritic  processes  line  the  sur- 
face of  the  dorsal  column  and  there  ramify:  the  axis-cylin- 
der process,  arising  sometimes  from  the  border  of  the  cor- 
puscles, sometimes  from  one  of  its  processes,  is  directed  ven- 
trally  across  the  substance  of  Rolando;  it  then  changes  its 
direction  to  reach  the  posterior  portion  of  the  lateral  col- 
umn, with  one  of  the  fibres  of  which  it  becomes  continuous. 

The  pyriform  or  fusiform  corpuscles  are  the  smallest 
elements  of  the  cord:  their  shape  is  quite  variable,  but 
forms  indicated  by  the  terms  above  are  prevalent.  It  is 
characteristic  of  them  that  they  are  elongated  dorsoven- 
trally  and  have  great  numbers  of  crooked  and  intermin- 
gled dendrites,  the  greater  portion  of  which  arise  from  a 
ventrally  directed  stalk  which  is  prolonged  almost  to  the 
head  of  the  dorsal  cornu.  The  axis-cylinder  process  gen- 
erally arises  from  the  posterior  portion  of  the  corpuscle 
and  passes  either  dorsally  or  laterally  to  become  continu- 
ous with  one  of  the  fibres  of  the  dorsal  column. 

The  stellate  corpuscles  are  situated  nearest  to  the  head 
of  the  dorsal  cornu:  they  unite  the  substance  of  Rolando 
with  that  region  by  means  of  their  abundant  spinous  den- 
drites. The  axis-cylinder  process  is  sometimes  directed 
lengthwise  of  the  cord;  it  then  comports  itself  as  a  por- 
tion of  a  short  process  corpuscle,  and  appears  to  end  in 
the  substance  of  Rolando  itself:  at  other  times  it  is  directed 
either  mesially,  to  become  continuous  with  a  fibre  of  the 
column  of  Burdach,  or  laterally,  to  form  a  slender  fibre  of 
the  marginal  zone  of  Lissauer. 


CIIAPTKK    XXII.      THE   NERVOUS  AXIS.  309 

Bv  a  CO  n():iris :)n  of  this  (Ijscription  of  the  corpuscles  as 
based  upon  the  destination  of  their  axis-cylinder  processes 
with  that  previously  j^iven  of  their  distribution  in  col- 
umnar tracts  aloni^  the  cord,  it  will  be  seen  that  these  lat- 
ter are  in  nearlv  everv  instance  composed  of  corpuscles  of 
v'lrvinar  relatiois.  n  )t  evi.';i  the  colu  n.is  of  the  ventral 
cornu  consistin'j^  solely  of  so  called  motor  corpuscles: 
Each  of  these  elon^jated  clusters  may  therefore  be  regarded 
as  consisting  of  corpuscles  so  coordinated  in  function  as  to 
justify  the  title  of  nuclear  or  ganglionic  columns  fre- 
quently applied  to  them. 

The  corpuscles  of  the  spinal  ganglia,  while  they  are 
situated  without  the  cord,  should  always  be  associated 
with  that  structure  in  any  attempt  to  form  a  complete 
conception  of  the  central  nervous  mechanism,  since  their 
eflferent  axis-cylinder  processes  enter  extensively  into  rela- 
tion with  the  dorsal  columns  andcornua.  Their  spheroidal 
or  ])vriform  contour  has  been  described  in  a  previouschap- 
ter,  as  has  also  the  manner  in  which  two  medullated  fibres 
are  derived  from  the  single  pole  or  process  borne  by  the 
bodv  of  the  corpuscle.  They  are  unique  in  the  fact  that 
one  of  these  fibres  puts  them  in  communication  with  den- 
dritic or  receptive  terminals  which  are  far  more  remote  in 
most  instances  than  those  of  any  other  nervous  element. 

The  central  canal  of  the  cord  is  lined  with  a  columnar 
epithelial  layer  of  ependyma  cells  which  are  frequently 
but  not  always  ciliated.  These  are  in  the  embryo  con- 
tinued b\'  slender  prolongations  which  reach  to  the  pia, 
forming  the  primary'  framework  of  the  cord  :  such  contin- 
uations persist  in  the  lower  vertebrates,  but  their  presence 
in  adult  birds  and  mammals  is  not  yet  well  established. 
In  connection  with  the  ependyma  cells,  and  possibly  de- 
rived therefrom,  there  are  found  in  the  adult  cord  of  the 


310  PART    If.      HISTOLOGICAL  ANATOMY.  , 

higher  vertebrates  numerous  neuroglia  cells  which  are 
particularly  abundant  in  the  white,  matter,,  immediatehr 
beneath  the  pi  a  and  along  the  septa  and  blood  vessels,  in 
the  substance  of  Rolando,  and  in  the  central  gelatinous, 
substance.  .  ,    >  ,         .       •  ,    ,  i 

The  blood  supply  of  the  cord  merits  special  mention.     A 
s;ingle  median  ventral  spinal  artery  (the  anterior  spinal 
of  human   anatomy)  runs  along  the .  ventral  margin  of. 
the  fold.of  the  pia  which  enter§  the  ventral  fissure:  while 
a  pair  of  dorsal  spinal  arteries  ;,(or  postejioj^  spinals). 
are    situated  just   ventrad   of   the    dorsal    roots    of   the 
spinal  nerves:  branches  of  these  arteries  ramify. in  the  ]')ia 
tK)    form   an,  extensive  plexus.     From  the,  ventral   tr,unkv 
small,  vessels,  the  central  arterioles  of  Ross,  injiumber 
several  times  as  many  as  the  iV,erte;brae  follow,  the  .fold  of, 
the  pia  into  the.median  fissure  as  far  as  the  white  commis-, 
sure.    Here  they  turn  altern^.tely  right ,  and'  left  to  reach: 
the  central  region  of  the  gray  matter  of,  either,  side*  where, 
they  breakup  into  small  arterioles  and  finally  into  capil; 
laries:  the  central  arterioles  are  distributed,  chiefly  to  the; 
gray  matter,  though  some  of  their  divisions  penetrate  thef 
white  ma,tter,  particularly  of  the  lateral  and  ventral  col- 
umns. '  , 

The  dorsa,l  vessels  and  the  pial  plexusgiyeoflT  great  num,- 
bers  of  peripheral  arterioles  which  follow  the  dorsal  me- 
dian septum  or  the  other  less  prominent  septa  into  the 
cord:  their  terminal  divisions^  are  .found  not  only  in  the 
w^hite  matter  but  also  in  the, outer  portion  of  the  grayi 
matter  and  throughout  the, dorsal  coruua.  Each  a.irteriole, 
whether  central  or  peripheral,  has  its  own  proper  capil-; 
lary  area,  anastomoses  between  adjacent  arterioles  not 
being  known  to  occur. 

Similarly  disposed  central  and  peripheral  venules 
o^rry  the  blood  from  the  capillary  networks  to  the  irregu- 


CHA<»fER   XXII.      TilK   NERtoUS  AXIS.  311 

Inr  plexuses  of  veins  in  the  ()ia  and  to  tlie  ])rinei|)al  venous 
trunks:  these  are  two  in  number,  a  ventral  which  follows 
the  ventral  artery  nmre  or  less  eloselv,  and  a  dorsal, 
\\  hieh overlies  the  niediiin  dorsal  septum  and  is  not  the 
compatiion  of  any  artery.  '      '  ' 


^'The  brain  is  the  mo'dified  and  specialized  anterior  end  of 
the  nervous  axis.  As  we  pass  from  the  spinal  cord  into 
the  medulla  oblonfrata,  and  thence;  throu<;h  the  region  of 
the  pons,  into  the  crura  cerebri,  we  find  that  the  fibrous 
tracts  which  can  be  recognized  in  the  white  matter  of  the 
coi^d  here  become  subdivided  and  varibusl}'  modified,  some 
of  theni  sooii  disappearing  as  such,  while  others  mav  be 
traced  almost  to  the  most  anterior  portions  of  the  brain; 
while  new  fibrous  tracts  variouslv  related,  mav  be  de- 
tected by  the  method  of  Waller  and  particularh'^  bv  that 
of  F'lechsig,  The  central  canal  expands  here  and  there  to 
form  the  ventricular  cavities  of  the  brain ;  while  its  epen^ 
dymal  lining  frequently  coming  in  contact  with  the  pial 
•investment  through  the  absence  of  intervening  nervous 
tissues;  is  often  thrown  into  A'^ascular  folds  of  greater  or 
less  exrterit.  The'gray  matter*,' which  ih  the  c6rd  sur- 
rounds the  ceritral  canal,  now  lies  chiefly  in  its  floor  and 
yides,  and  is  penetrated  and  subdivided  by  the  diversified 
fibre  tracts  already  referred  to.  The  ganglionic  columns, 
■v^'hich  in  the  cord  were  continuous  throughout  the  whole 
or  large  portions  of  its  structure,  are  now  broken  up  into 
more  or  less  definite  nuclear  aggregates  of  corpuscles,  ser- 
ving a«i  centres  for  specific  cranial  nerves:  some  of  these 
nuclei  may  perhaps  be  homologized  with  portions  of  some 
of  the  ganglionic  columns ;  while  for  others  no  such  rela- 
tion is  discernible. 

In  addition  to  this  central  prolongation  and  modifica- 
tion of  the  cord,  other  structures  appear  connected  there 


312  PART  II.      HISTOLOGICAL   ANATOMY. 

with  Nvliicli.  while  they  must  unquestionably  be  regarded 
as  developments  of  the  axial  region,  are  of  such  size  and 
structural  importance  as  to  be  properh'  regarded  as  sub- 
stantially additions  thereto.  These,  like  the  cord,  and  the 
basal  portions  of  the  brain  as  well,  are  composed  of  more 
or  less  definite  fibre  tracts  and  corpuscular  areas,  related 
to  each  other  and  to  the  more  axial  structures. 

Within  recent  years  great  progress  has  been  made  in  the 
process  of  unraveling  this  complex  structure,  and  it  is 
safe  to  say  that  the  general  topography  of  the  brain  is 
known.  To  attempt  to  sketch  it.  however,  would  take 
far  more  space  than  the  range  of  the  present  work  con- 
templates, and  would  take  us  largeh^  into  a  field  where, 
while  the  method  has  been  and  must  be  chiefl\'  that  of  the 
histologist,  the  results  belong  rather  to  the  domain  of  the 
anatomist.  Aledullated  fibres,  axis-cylinder  processes  with 
their  collaterals,  and  multipolar  corpuscles  are  much  the 
same  in  appearance  and  in  relations  throughout  a  large 
portion  of  the  brain  as  in  the  spinal  cord  :  and  we  are  con- 
cerned from  a  histological  standpoint  only  with  those 
resions  of  the  brain  in  which  new  forms  of  nervous  ele- 
m.ents  appear  or  in  w-hich  some  special  mode  of  combina- 
tion is  demonstrable.  The  chief  of  these  are  the  cerebel- 
lum, the  cerebral  hemispheres,  and  the  olfactory  bulbs ; 
an  account  of  the  cerebellum  and  of  the  hemispheres  will 
now  be  given  :  that  of  the  olfactory  bulb  will  be  deferred 
until  the  sense  organ  with  which  it  is  connected  has  been 
described. 

The  cerebellar  cortex  is  a  superficial  la^^er  of  gray  mat- 
ter whose  well  marked  folds  form  the  laminae  visible  on 
the  surface  of  the  organ  to  the  naked  eye:  the  middle  of 
each  fold  is  occupied  b}'  a  mass  of  white  fibres  in  direct 
relation  to  the  gray  matter.  The  latter  shows  to  the 
naked  eye  tw^o  distinct  strata ;  an  outer  and  paler  known 


CHAPTER    XXII.      THK   NERVOUS  AXIS.  313 

as  the  molecular  layer,  and  an  inner,  of  a  rusty  brown 
color,  called  the  granular  layer.  Between  these,  and  par- 
tially imbedded  in  each,  is  a  nearly  continuous  stratum  of 
kir;j[e  corpuscles,  the  most  characteristic  of  the  cerebellar 
cortex,  the  corpuscles  of  Purkinje.  They  are  pyriform  or 
flask-shaped,  the  large  extremity  being  directed  inward: 
from  the  latter  an  axis-cylinder  process  is  given  oft'  which 
passes  through  thegranular  layer  to  enter  the  white  mat- 
ter as  'ci  medullated  nerve  fibre;  during  its  course  through 
the  granular  layer  it  gives  off^  collaterals  which  in  many 
cases  turn  backward  to  enter  the  molecular  layer. 

The  outer  extremity  of  the  corpuscle  is  prolonged  for  a 
greater  or  less  distance,  but  usually  soon  divides  into  two 
])rincipal  branches,  which  rapidly  and  repeatedly  subdi- 
vide to  give  rise  to  large  numbers  of  dendritic  processes, 
many  of  which  are  continued  to  the  surface  of  the  cortex: 
their  surface  is  beset  with  .short  processes  which  end 
bluntly.  The  ramification  is  in  every  case  almost  entireh' 
confined  to  a  plane  transverse  to  the  lamella  in  which  the 
corpuscle  is  situated. 

The  great  majority  of  the  corpuscles  of  the  granular 
layer  are  exceedingly  small,  with  large  nuclei  and  very 
scanty  surrounding  protoplasm :  their  great  numbers, 
and  their  appearance  when  stained  with  carmine  or  other 
similar  stains  led  to  the  name  above  given  for  the  region 
in  which  the}'  occur.  For  a  long  time  their  nervous  char- 
acter was  doubted  or  denied.  The  more  recent  technical 
methods  have  demonstrated  it  beyond  question,  and  the}' 
are  now  known  as  granule  corpuscles.  Their  bodies  con- 
tain a  relatively  large  quantity  of  rusty  brown  pigment, 
to  which  the  characteristic  color  of  the  layer  is  due,  A 
few  protoplasmic  processes  are  given  off  b\'  each  corpus- 
cle: these  branch  sparingly  to  end  in  a  sm.all  number  of 
dendrites  with  thickened  extremities.  From  the  bod}'  of 
the  corpuscle,  or  frequently  from  one  of  the  processes,  a 


314  PART   II.      HISTOLOGICAT.   ANATOMY. 

slender  axis-cylinder  process  is  given  off,  which  passes 
without  collaterals  into  the  molecular  layer  and  there 
undergoes  a  T-division  to  form  two  slender  tangential 
fibres  whose  course  is  alwa^'s  in  the  direction  of  the 
lamella  in  which  the}^  occur  and  therefore  at  right  angles 
to  the  plane  of  the  dendrites  of  the  corpuscles  of  Purkinje, 
with  which  they  are  in  close  contact  as  the\^  pass.  The 
tangential  fibres  have  been  shown  in  some  of  the  smaller 
vertebrates  to  run  the  whole  length  of  the  lamellae  in 
which  they  are  situated. 

In  addition  to  the  small  granule  corpuscles  there  are 
present  in  the  inner  layer,  though  in  small  numbers,  other 
nervous  elements.  They  are  situated  near  the  outer  limit 
of  the  layer  and  are  nearly  as  large  as  the  corpuscles  of 
Purkinje,  which  they  somewhat  resemble  in  form.  Their 
outer  region  sends  off  numerous  protoplasmic  processes, 
which  branch  irregularh^  in  every  direction  to  form  large 
numbers  of  dendrites:  these  project  chiefly  into  the  mole- 
cular layer,  though  many  of  them  lie  altogether  within 
the  granular  layer.  From  the  inner  region  there  is  given  off 
a  slender  axis-cylinder  process,  which  branches  freelj'- 
almost  from  its  origin,  the  whole  giving  rise  to  an  exten- 
sive arborization-plexus  which  is  situated  entirel^within 
the  granular  layer.  These  corpuscles  therefore  opb^ng  to 
Golgi's  second  type. 

The  finely  dotted  appearance  to  which  the  molecular 
layer  owes  its  name,  seen  when  the  lamella  is  cut  trans- 
versely, is  largely  due  to  the  cut  ends  of  the  tangential 
fibres.  There  are  present  in  this  layer  two  kinds  of  cor- 
puscles. In  the  deeper  portion  are  seen  numerous  corpus- 
cles of  medium  size  and  irregular  form,  whose  dendritic 
processes  branch  sparingly  but  extend  for  some  distance 
into  the  surrounding  region  :  the  axis-cylinder  process 
takes  a  course  generally  parallel  to  the  surface  of  the  cor- 
tex and  gives  off  frequent  collaterals ;  these  pass  inward 


CIIAPTKK    XXII.      TIIK    NKK VOl'S  AXIS.  315 

to  be  ji|)|)lic(l  in  each  instance  to  the  bodv  of  a  cor- 
puscle of  Puikinje,  ujjon  whicii,  or  about  the  base  of  the 
axis-cylinder  process,  tliev  terminate  with  little  if  anv  snb- 
(bvision:  a»nunil)er  of  such  fibrils  surround  each  corjjus- 
cle  of  Purkinje,  forming  a  nest  or  basket  about  it:  the 
cor|)uscles  Ironi  which  they  proceed  are  therefore  known 
as  basket  corpuscles. 

Throus^hout  the  molecular  laver,  but  chiefly  in  the  outer 
portion  thereof,  are  found  numerous  stellate  COrpuscles, 
which  though  smaller  in  size,  resemble  the  elements  just 
described  in  their  general  form  and  in  the  appearance  of 
their  sparingly  brdnched  dendrites.  The  destination  of 
their  axis-cylinder  processes  is  not  known 

The  axis-cylinder  processes  of  the  corpuscles  of  Purkinje 
give  rise  to  the  only  fibres  of  the  central  white  matter  of 
the  lamella  known  to  be  sent  inward  from  the  cerebellar 
cortex.  The  fibres  which  pass  out  into  it  have  been  shown 
to  terminate  in  two  different  methods.  Some  of  them  end 
in  the  granular  layer,  where  their  extremities  branch  spar- 
ingly, the  subdivisions  terminating  in  enlargements  in 
such  a  way  as  to  give  to  the  whole  somewhat  the  appear- 
ance of  a  tuft  of  moss:  the}' have  therefore  been  desig- 
nated EQQSSy  fibres.  The  others  find  their  way  to  the  cor- 
puscles of  Purkinje,  traverse  their  surfaces,  and  subdivide 
to  follow  the  branchings  of  the  dendritic  processes,  thus 
forming  a  terminal  arborization  which  adheres  thereto 
like  a  vine  to  a  tree:  they  have  therefore  received  the  name 
of  climbing  fibres. 

It  will  be  seen  from  the  above  description  of  the  cerebel- 
lar cortex  that  each  corpuscle  of  Purkinje  is  in  relation 
with  three  sets  of  discharging  terminals:  the  tangential 
fibres  of  the  granule  corpuscles,  the  collaterals  of  the  bas- 
ket corpuscles,  and  the  arborizations  of  the  climbing 
fibres.  We  are  aX  present  entirel)'  ignorant  of  the  func- 
tional relations  which  are  based  on  this  structure. 


316  PART    II.       HISTOLOGICAL   ANATOMY. 

The  cerebral  cortex  has  been  regarded  as  composed  of 
several  distinct  la^-ers  which  differ  from  each  other  as  re- 
gards the  form  and  size  of  the  contained  corjDuscles.  As 
our  knowledge  has  increased,  the  boundaries  between 
these  la\'ers  have  been  found  to  be  less  sharply  defined 
than  was  at  first  supposed.  Three  can  with  certainty  be 
distinguished:  the  outer,  or  so  called  molecular  laver,  the 
middle,  or  p3'ramidal  laj-er,  and  the  inner,  or  polymor- 
phous la^er. 

The  molecular  layer,  like  that  of  the  cerebellar  cortex, 
owes  its  characteristic  appearance  largely  to  the  cut  ends 
of  the  collaterals  and  slender  fibres  which  are  denselv  in- 
terwoven in  it  and  to  their  terminals.  Its  outermost  por- 
tion contains  numerous  neuroglia  cells,  which  just  beneath 
the  pia  form  an  almost  continuous  stratum,  as  in  the 
spinal  cord:  this  has  been  described  as  a  distinct  layer  of 
the  cortex.  vScattered  throughout  the  molecular  layer  are 
numerous  nervous  elements,  the  corpuscles  of  Cajal,  that 
histologist  having  first  demonstrated  their  distinguishing 
characteristics.  Two  forms  have  been  described  b}^  him, 
the  fusiform  and  the  stellate.  The  former  are,  as  their 
name  implies,  spindle  shaped,  and  give  off  from  either  end 
a  polar  process  which  runs  parallel  to  the  surface  of  the 
cortex :  the  two  processes  cannot  be  distinguished  struc- 
turalh'-,  and  each  may  take  on  the  character  of  an  axis- 
cylinder  process :  from  each  collaterals  are  given  off  at 
right  angles  or  nearly  so,  which  are  invariablj''  directed 
toward  the  surface  of  the  cortex.  In  the  second  form  the 
number  of  similar  processes  is  increased  to  three  or  more. 
In  each  case  the  processes  terminate  eventually  in  ramifi- 
cations which  are  turned  toward  the  cortical  surface. 

Cajal  has  also  described  in  the  molecular  la^^er  a  third 
form  of  corpuscle  under  the  name  of  polygonal;  these 
have  several  protoplasmic  processes  which  end  in  dendrites 


CHAPTER    XXII.      THP:   NERVOUS  AXIS.  317 

which  nuiv  extend  beyond  the  molecular  layer  to  enter 
that  beneath  it.  The  axis-cylinder  process  may  arise 
either  from  the  body  of  the  corpuscle  or  from  one  of  the 
processes:  its  terminal  subdivisions  are  confined  to  the 
molecular  layer. 

The  pyramidal  layer  is  very  commonly  divided  into  two 
strata,  the  layer  of  small  pyramids,  next  the  molecular 
layer,  and  the  layer  of  large  pyramids,  immediately  sub- 
jacent. The  elements  characteristic  of  these  two  layers 
are,  however,  so  nearly  alike  in  everything  but  size,  and 
the  transition  from  the  one  to  the  other  is  so  gradual  in 
this  respect,  that  they  mav.  at  least  for  the  present,  be 
advMutagcouslv  considered  as  one. 

The  most  numerous  of  the  elements  peculiar  to  this  layer 
are  the  pyramidal  corpuscles:  their  form  is  indicated  by 
their  name.  The  base  of  the  pyramid  is  turned  away  from 
the  surface  of  the  cortex,  the  apex  being  directed  verti- 
cally upward  and  continued  into  a  long  tapering  ascend- 
ing stem  which,  even  from  the  corpuscles  most  deeply 
placed,  extends  nearly  or  quite  to  the  molecular  layer:  it 
terminates  by  subdivisions  into  a  tuft  of  protoplasmic 
processes;  similar  processes  are  given  off"  at  right  angles 
along  its  course;  while  others  are  given  oft'  from  the  bodv 
of  the  corpuscle,  and  particularly  from  the  angles  of  its 
base :  all  these  processes  are  probably  to  be  regarded  as 
dendritic :  their  subdivisions  are  distributed  chiefly  to  the 
surrounding  substance  of  the  pyramidal  layer,  save  those  of 
the  apical  tuft,  which  are  largely  situated  in  the  molecular 
layer.  An  axis-cylinder  process  is  given  off"  from  the  base 
of  each  pj-ramidal  corpuscle,  usualU' from  a  point  near  the 
centre:  it  is  always  directed  toward  the  white  matter  be- 
neath the  cortex:  collaterals  are,  however,  given  oft"  while 
it  is  still  in  the  gray  matter,  some  of  which  run  horizon- 
tally to  terminal    ramifications    within    the    pyramidal 


318  PART   11.      HISTOLOGICAL   ANATOMY. 

layer,  while  others  bend  upward  at  a  right  angle  to  termi- 
nate in  the  molecular  layer. 

Within  the  lower  portion  of  the  pyramidal  layer  are  cdso 
found,  scattered  here  and  there,  certain  elements  confined 
to  the  cerebral  cortex,  the  carpuscles  of  Martinotti :  they 
are  also  found  in  the  third  or  polymorphous  layer.  They 
are  spindle  shaped,  roughly  pyramidal,  or  irregular  in 
form,  their  distinguishing  characteristic  being  the  distribu- 
tion of  the  dendritic  processes  chiefly  outward  and  down- 
ward, and  an  axis-cylinder  process  which  generally  arises 
from  the  uppermost  portion  of  the  corpuscle,  though  some- 
times from  one  of  the  ascending  protoplasmic  processes, 
and  ascends  toward  the  molecular  layer  in  which  its  rami- 
fications are  usually  situated :  in  some  instances  the  ter- 
minals lie  wholly  or  in  part  in  the  uppermost  portion  of 
the  pyramidal  layer. 

The  polymorphous  layer  is  so  called  from  the  occur- 
rence therein  of  elements  which  vary  greatly  in  form  as  well 
as  in  size:  they  may  be  ovoid,  spindle  shaped,  pyramidal, 
or  polygonal.  They  agree,  however,  in  the  fact  that  their 
long  axes  are  as  a  rule  disposed  horizontally  to  the  sur- 
face of  the  cortex,  and  that  when  a  terminal  or  apical 
stalk  is  present  it  is  never  vertically  directed  as  in  the 
pyramidal  layer.  Some  of  the  elements  present  are,  as 
has  just  been  indicated,  ascending  corpuscles  of  Mar- 
tinotti: others  belong  to  the  second  type  of  Golgi,  having 
short  axis-cylinder  processes  which  break  upinto  terminal 
ramifications  in  the  immediate  vicinity  of  the  corpuscle. 
Still  others  give  off  long  axis-cylinder  processes  which 
bend  downward  to  enter  the  white  matter  and  become 
medullated  fibres  of  varying  distribution. 

The  deeper  portion  of  the  polymorphous  laj'cr  contains 
chiefly  small  fusiform  corpuscles,  which  has  led  to  its  dis- 
tinction en  the  part  of  some  histologists  as  a  separate 


,     cnAPTKK  xxii.    Tin-:  nkkvois  axis.  319 

hiver:  tliis  is  more  clearly  deHiied  in  the  rej^ioii  of  the 
island  of  Rjil  than  elsevvh.'re,  where  the  strataiii  in  (|ucs- 
tion  is  separated  from  the  rest  of  the  cortex  by  intervening 
white  matter,  forminf:^  the  layer  visible  to  the  naked  eye, 
known  as  the  claustl'um.  In  most  portions  of  the  cortex 
the  stratum  in  cjiu'stion  is  not  clearlv  definable  from  the 
rest  of  the  ])olymorphous  layer. 

Brief  mention  may  perhaps  l)e  made  of  the  composition 
of  the  subjacent  white  fibrous  layer,  although  the  vari- 
ous fibres  and  tracts  are  not  histologically  distinguishable, 
save  to  a  certain  extent  by  the  methods  of  Waller  and  of 
Flechsig.  Fibres  formed  by  the  development  of  medullary 
sheaths  about  axis-cylinder  processes  which  descend  from 
the  corpuscles  of  the  cortical  gray  matter  may  pass  on 
downward  as  projection  fibres  to  the  basal  ganglia,  the 
hindbrain,  or  the  spinal  cord  itself:  other  fibres  pass  as 
association  fibres  to  other  ])ortions,  more  or  less  remote, 
of  the  cortex  of  the  same  hemisphere:  while  others  still 
pass,  chiefly  by  way  of  the  corpus  callosum,  as  commis- 
sural fibres  to  regions  of  the  cortex  of  the  opposite  hemi- 
si)here;  not  unfrequently  an  axis-cylinder  process  may  be- 
come a  projection  or  an  association  fibre,  and  one  or  more 
of  its  collaterals  an  association  or  a  commissural  fibre: 
or  the  converse  may  occur.  Still  other  fibres  pass  upward 
into  the  cortex  by  wa}'  of  projection,  association,  or  com- 
missural tracts  to  end  there,  the  terminal  arborizations 
being  situated  either  in  the  molecular  or  the  upper  portion 
of  the  pyramidal  layer. 

The  neuroglia  of  the  brain  does  not  differ  from  that  of 
the  spinal  cord  to  such  an  extent  as  to  merit  a  detailed 
description  in  so  brief  an  account  of  the  organ  as  is  here 
given.  Much  the  same  mav  be  said  of  the  ependyma, 
which  lines  the  ventricles  and  passage  ways  of  the  brain : 


320  PART  II.       HISTOLOGICAL   ANATOMY. 

like  that  of  the  central  canal  of  the  cord,  it  is  composed 
chiefly  of  columnar  cells,  whose  free  extremities  bear  for  a 
time  at  least  cilia-like  processes  not  known  to  be  vibratile. 
Mention  should  be  made  of  the  plexuses  of  the  ventricles, 
formed  b}^  infoldings  of  the  pia  and  the  ependyma,  and 
consisting  chiefly  of  a  rich  network  of  small  bloodvessels 
supported  by  the  former  and  invested  b}'  the  latter.  An 
outline  of  the  blood  supply  of  the  cord  was  given  above: 
that  of  the  brain  is  far  too  complex  and  too  much  a  mat- 
ter of  gross  anatomy  to  be  described  here.  Neither  will 
any  account  be  attempted  in  this  connection  of  the  devel- 
opmental history  of  the  nervous  axis,  beyond  the  state- 
ment that  it  is  formed  entirely  from  an  infolding  of  the 
outer  or  epiblastic  layer. 


CHAPTER    XXIII.      Tlin  ORGANS  OF  SPECIAL  SENSE.    321 


CHAPTER  XXIir. 
THE  ORGANS  OF  SPECIAL  SENSE. 


As  was  stated  in  a  previous  chapter,  there  are  certain 
organs  in  which  specially  modified  receiving  terminals  are 
associated  with  more  or  less  highly  modified  forms  of  epi- 
thelium and  with  other  special  structures  of  a  skeletal 
character  to  form  in  each  case  an  apparatus  for  the  recep- 
tion of  a  specific  and  clearly  defined  impression ;  their 
stimulation  giving  rise  to  sensations  of  flavor,  odor, 
sound,  or  light,  commonly  called  special,  as  distinguished 
from  the  more  diffused  and  less  clearly  definable  sensations 
of  temperature,  contact,  resistance,  etc.,  received  by  the 
more  widely  scattered  and  possibly  less  specialized  termi- 
nals described  in  the  chapter  referred  to. 

In  each  case  the  special  receiving  apparatus  involved  has 
associated  therewith  other  special  structures,  chiefly  skele- 
tal, whose  function  it  is  to  render  more  intense  or  more 
specific  the  impression  received:  these  associated  struc- 
tures being,  equally  with  the  nervous  apparatus  in  ques- 
tion, essential  factors  of  the  organ  of  special  sense.  As  in 
the  case  of  the  brain,  a  full  description  of  these  structures 
belongs  rather  to  the  province  of  anatomy  than  to  that  of 
histolog}',  and  would  require  far  more  space  than  can  with 
propriety  be  given  here:  an  account  of  the  histological 
composition  of  the  essential  apparatus  will  in  each  case 
be  given,  together  with  mention  of  any  characteristic 
features  noteworthy  in  the  tissues  of  the  accessory  parts, 
some  previous  knowledge  of  the  anatomy  of  the  organs 
in  question  being  presumed. 


322  PART   II.      HISTOLOGICAL    ANATOMY. 

The  immediate  organs  of  taste  are  the  taste-buds,  so 
called  from  their  spheroidal  form,  which  are  situated  in 
large  numbers  upon  both  the  outer  and  the  inner  sides  of 
the  valle\'S  which  surround  the  circum vallate  papillae;  on 
the  fungiform  papillae;  and  particularly  upon  the  loose 
folds  just  in  front  of  the  anterior  pillars  of  the  fauces  which 
in  inan  represent  the  more  definiteU'  circumscribed  foliate 
papillae  of  some  of  the  lower  mammals:  they  are  also 
found  on  the  soft  palate  and  the  epiglottis,  and  arc  scat- 
tered here  and  there  over  the  surface  of  the  tongue.  They 
are  spheroidal  bodies,  almost  completely  embedded  in  the 
stratified  squamous  epithelium  of  the  surface  where  the\' 
occur:  the  long  axis  is  directed  vertically  or  nearly  so  to 
the  surface,  and  the  outer  extreinitv  tapers  slightly,  on 
which  account  their  form  is  sometimes  described  as  flask 
shaped.  The  mass  consists  of  a  number  of  elongated  epi- 
thelial cells,  of  which  some  are  spindle  shaped,  or  flattened, 
and  are  known  as  sustentacular  cells :  a  layer  of  these 
completed  covers  the  outer  surface,  their  grouping  recall- 
ing somewhat  the  surface  segmentation  of  acantelope: 
others  are  scattered  irregularly  throughout  the  interior. 
Between  them  lie  other  cells  whose  bodies,  except  just 
around  the  large  nucleus,  are  slender  and  almost  filamen- 
tous; these  are  the  so  called  gustatory  cells.  The  outer 
extremity  of  each  ends  in  a  ciliary  process,  the  taste-hair, 
which  projects  with  its  fellows  through  a  small  circular 
opening  in  the  squamous  epithelium  known  as  the  gus- 
tatory pore:  its  inner  extremity  is  slender,  often  bifur- 
cated, and  frequently  more  extensively  branched  ;  its  sub- 
divisions, which  are  sometimes  varicose,  reach  to  the  base 
of  the  taste-bud. 

Numerous  attempts  have  been  made  to  demonstrate  a 
structural  relation  between  theelements  just  described  and 
the  nerve  fibres  which  pass  to  the  taste-buds  from  the  sub- 
divisions of  the  glossopharyngeal    nerve,   but    thus    far 


CH.VPTKK    XXIII.      TlIK  ORCANS  OF  SPKCIAI,  SENSE.      323 

without  success.  This  ucrvc,  like  the  dorsal  root  of  a 
spinal  nerve,  bears  a  j^anglion  near  its  point  of  union  with 
the  nervous  axis:  examination  by  the  chromate  and  silver 
method  shows  that  tiie  axis  cylinders  of  the  nerve  fibres 
end  in  the  taste-buds  by  branching  ainoni^f  the  cells  in  the 
interior,  forniin*,^  what  are  known  asintrabulbar  ramifica- 
tions: the  honiolosi^y  existing  between  the  glossopharvn- 
jjeal  and  the  spinal  nerves  would  indicate  that  these  are  to 
be  regarded  as  dendritic  processes  at  the  ends  of  long  af- 
ferent fibres,  similar  to  the  "free  endings"  in  the  epidermis 
described  in  the  chapter  on  the  nervous  tissues.  Accord- 
ing to  this  view  it  is  questionable  whether  the  so  called 
gustatory  cells  are  in  reality  nervous  in  character,  and 
some  have  regarded  them  and  the  sustentacular  cells  as 
alike  modified  epithelial  elements  whose  form  and  arrange- 
ment favors  the  stimulation  of  the  nerve  terminals.  There 
is,  however,  a  close  relationship  between  the  senses  of 
taste  and  of  smell ;  and  the  structure  of  the  receiving  ap- 
paratus of  the  latter  suggests  an  explanation  of  the  struc- 
ture of  the  taste-buds  in  which  the  gustatory  cell  would 
form  the  first  member  in  a  series  of  nervous  elements. 
Reference  will  be  made  to  this  after  the  organ  of  smell  has 
been  described:  the  statements  already  made,  however, 
represent  the  present  extent  of  our  knowledge  of  the  facts 
in  the  case. 

Mention  should  be  made  here  of  the  fact  that  some  of 
the  medulla  ted  nerve  fibres  of  the  subdivisions  of  the 
glossopharyngeal  nerve  going  to  the  taste-buds  terminate 
bv  free  endings  in  the  stratified  epithelium  immediately 
surrounding  those  structures,  forming  what  are  termed 
peribulbar  ramifications :  these  are  generally  regarded  as 
fibres  of  general  and  not  of  special  sensibilitv.  The  glands 
of  a  serous  type,  known  as  the  glands  of  Ebner,  which 
are  closely  associated  with  the  taste-buds,  have  been  de- 


324  PART   ir.      HISTOLOGICAL   ANATOMY. 

scribed  in  connection  with  the  tongue.  They  are  to  be 
distinctly  regarded  as  accessor}^  to  the  apparatus  of  taste 
perception,  the  fluid  secreted  by  them  aiding  in  the  solu- 
tion of  substances  whose  flavor  is  to  be  perceived  by  the 
taste-buds. 


The  sense  of  taste  resembles  most  forms  of  general  bodih- 
sensation  in  requiring  the  actual  contact  of  the  object  per- 
ceived: the  remaining  special  senses  resemble  the  thermal 
sense  in  being  capable  of  givingknowledge  of  objects  at  a 
distance.  Of  these  the  first  and  as  regards  its  receiving 
mechanism  the  simplest  is  that  of  smell.  The  accessory- 
muscular  and  skeletal  structures  which  make  up  the  facial 
region  known  as  the  nose  require  no  special  description 
from  a  histological  standpoint.  The  air  passages  which 
they  enclose  are  lined  with  a  mucous  membrane  known  as 
the  pituitary  or  the  Schneiderian  membrane :  the  vesti- 
bule, into  which  the  nostril  opens  on  either  side,  is  lined 
with  stratified  squamous  epithelium  continuous  at  the 
margin  of  the  nostril  with  the  epidermis,  of  which  it  is  a 
modification  :  the  remainder  is  divided  into  two  portions, 
the  lower  or  respiratory  and  the  upper  or  olfactory  :  the 
former  is  lined  with  stratified  ciliated  columnar  epithelium, 
similar  to  that  of  other  respiratory^  passages,  beneath 
which  is  a  highly  vascular  membrane  which  contains  a 
considerable  amount  of  adenoid  tissue  here  and  there  gath- 
ered into  distinct  nodules,  and  numerous  racemose  glands, 
some  of  which  are  mucous  and  others  are  serous  in  char- 
acter: large  numbers  of  goblet  cells  are  also  distributed 
throughout  the  epithelium. 

The  olfactory  region  of  the  nasal  mucosa  can  be  distin- 
guished with  the  naked  eye  b}'  means  of  its  well  marked 
pigmentation,  it  being  of  a  yellow  color  in  man  and  some 


CHAPTER    XXIII,     THE  ORGANS  OF  SPECIAL  SENSE.     325 

of  the  lower  mnmmals,  nnd  of  a  yellowish  brown  in 
others.  The  fibrous  layer  is  more  highly  vascular  than  in 
the  respiratory  region,  but  contains  less  adenoid  tissue:  it 
contains  numerous  glands,  the  glands  of  Bowman,  which 
diflTer  from  the  racemose  glands  above  mentioned  in  being 
tubular,  rarely  branched,  and  but  slightly  bent  or  convo- 
luted: the  distal  extremity  is  frequently  the  largest,  the 
tube  tapering  toward  the  duct,  which  is  always  slender, 
and  opens  either  upon  the  mucous  surface  or  occasionally 
into  a  small  ciliated  crypt:  the  epithelium  of  the  glands 
of  Bowman  is  of  the  serous  type,  but  the  tubules  resem- 
ble those  of  mucous  glands  in  having  a  conspicuous  lumen: 
ordinary  racemose  glands  are  also  occasionally  found  in 
the  olfactory  region. 

The  epithelial  layer  of  the  olfactor}'  region  is  composed 
chiefly  of  two  kinds  of  elements.  The  first  comprises  the 
non-ciliated  columnar  supporting  cells:  these  are  chiefly 
prismatic  in  form  throughout  the  greater  portion  of  their 
extent,  but  with  tapering  inner  extremities,  and  with  oval 
nuclei  situated  at  an  approximateK' uniform  distance  from 
the  surface:  other  more  deeply  situated  epithelial  cells  are 
])yramidal  in  form,  their  bases  resting  on  the  fibrous  layer; 
they  ma\'  perhaps  be  regarded  as  immature  supporting 
cells.  Interspersed  among  the  columnar  epithelial  cells  are 
large  mambers  of  slender  elements  of  the  second  kind, 
whose  outer  extremities  terminate  in  tufts  of  hair-like  pro- 
cesses, the  olfactory  hairs,  which  project  above  thegeneral 
surface:  the  middle  portion  is  suddenly  thickened  to  con- 
tain large  spheroidal  nuclei :  these  are  the  olfactory  cells: 
their  slender  varicose  inner  portions  are  now  known  to  be 
continuous  with  the  medullated  fibres  of  the  nerves  of 
smell.  They  must  therefore  be  regarded  as  nervous  ele- 
ments, the  thickened  middle  portion  which  contains  the 
nucleus  constituting  the  bod\'  of  the  corpuscle,  and  the 
peripheral  portion  a  greatly  reduced  dendritic  region  con' 


326  Part  ii.    histological  anatomy. 

sivSting  of  a  single  protoplasmic  process;  while  the  proxi- 
mal portion  passes  over  into  an  axis-cylinder  process 
which  shortly  becomes  a  non-meclnllated  nerve  fibre. 

The  olfactory  nerve-fibres  can  bo  follov^^ed  through  the 
cribriform  plate  to  their  passage  into  the  surface  of  the 
olfactory  bulbs,  whose  structure  may  now  be  considered  : 
as  has  been  stated  in  a  previous  chapter,  the^^  constitute 
a  distinct  region  of  the  brain;  but  their  histological  struc- 
ture is  so  intimately  associated  with  their  relation  to  the 
sense  of  smell  as  to  make  their  description  appropriate  in 
this  connection.  Each  olfactory  bulb  is  a  rounded  mass 
at  the  anterior  extremity  of  the  longer  or  shorter  olfac- 
tory tract  (or  olfactory  nerve,  improperly  so  called):  the 
whole  is  an  outgrowth  from  the  hemisphere  and  originally 
contains  a  cavity,  the  olfactory  ventricle,  which  is  a  di- 
verticulum of  the  lateral  ventricle:  in  many  mammals  this 
cavity  persists  throughout  life ;  in  some  it  persists  in  the 
bulb,  that  of  the  tract  being  obliterated ;  in  man  and  the 
Primates  generally  it  disappears  altogether  in  the  adult. 

In  passing  across  a  section  of  the  olfactory  bulb  from 
the  surface  in  close  proximity  to  the  cribriform  plate  to  the 
ependymal  lining  of  the  olfactory  ventricle  a  more  or  less 
distinct  stratification  may  be  observed:  the  number  of 
layers  distinguished  by  difterent  observers  varies  accord- 
ing to  the  degree  of  subdivision  recognized:  Cajal  desig- 
nates five,  distinguished  by  histological  characteristics  de- 
monstrable with  the  aid  of  the  silver  chromate  method,  in 
addition  to  the  ependymal  la\'er,  which  is  wanting  in 
those  forms  in  which  the  bulb  is  solid. 

The  first  of  these  is  the  superficial  layer  of  nerve 
fibres:  this  is  a  thin  stratum  of  slender  non-medullated 
fibres  arranged  in  a  felted  mass:  it  is  composed  exclu- 
sively of  the  constituents  of  the  bundles  which  pass 
through  the  perforations  of  the  cribriform  plate,  whose 
origin,  as  we  have  seen,  is  in  the  axis-cylinder  processes  of 


CllAPTHK  XXIll.     Tin:  ORGANS  OF  SPFXIAI.  SKNSK.      327 

ihc  olfactory  cells.  The  fibres  leave  the  layer  inwardly 
cither  siniilv  or  in  stnall  *;roups  to  enter  the  second  stra- 
lurn.  or  layer  of  olfactory  glomeruli:  the  bodies  whose 
presence  distin«i:uishcs  this  layer  have  lon<;  been  known  to 
histolosiists.as  spheroidal  masses  present  inlarji^e  nnnibers 
near  the  surface  of  the  bulb:  it  is  only  recently  that  their 
structure  has  been  at  all  understood :  they  are  composed 
in])arL  hv  the  dense  tufts  of  varicose  fibrils  which  form  the 
terminal  arborizations  of  the  olfactory  fibres  entering 
them  from  the  superficial  layer;  in  part  by  the  similarly 
tufted  dendritic  ramifications  of  the  extremities  of  pro- 
cesses derived  from  corpuscles  situated  in  a  deeper  layer 
to  be  presently  described:  they  are,  therefore,  the  places 
where  ingoing  impulses  are  transmitted  from  the  first  to 
the  second  of  a  series  of  nervous  elements. 

The  third  stratum  is  termed  by  Cajal  the  molecular 
layer:  as  is  the  case  with  other  structures  similarly  desig- 
nated in  various  parts  of  the  nervous  axis,  the  finely 
punctate  appearance  which  characterizes  it  when  seen  in 
section  is  due  to  the  cut  ends  of  numerous  fibres,  and  of 
fibre-like  processes  from  the  corpuscles  of  the  layer  next 
adjacent.  Cajal  describes  in  addition,  as  i)cculiar  to  this 
layer,  certain  elongated  or  fusiform  corpuscles  whose 
peripheral  extremities  are  continued  by  slender  processes 
which  run  to  the  glomeruli  and  there  terminate  in  small 
dendritic  tufts  which  are  subsidiary  to  those  of  the  cor 
puscles  of  the  layer  next  within:  their  proximal  extremi- 
ties give  rise  to  axis-cylinder  processes  which  run  to  the 
innermost  layer  and  there  bend  strongly  to  pass  toward 
the  olfactory  tracts,  which  they  enter  as  medullated 
fibres. 

The  layer  of  mitral  corpuscles  is  the  fourth  of  the  suc- 
cessive zones  :  it  consists  of  large  nervous  elements  chiefly 
disposed  in  a  sitigle  stratum,  whose  general  form  is  indi- 
cated by  their  title.    The  base  of  each  corpuscle  is  directed 


328  PART  rr.    histological  anatomv. 

toward  the  outer  surface  of  the  bulb:  it  gives  off,  in  mam- 
mals usually  from  a  point  near  its  centre,a  stout  descend- 
ing process  which  traverses  the  molecular  la^^er  to  form 
in  one  of  the  glomeruli  the  important  dendritic  ramifica- 
tion already  described  as  one  of  the  essential  constituents 
of  each  of  those  bodies.  From  the  margin  of  the  base  are 
given  off  stout  protoplasmic  processes  which  diverge 
greatly,  their  ramifications  interlacing  to  form  a  layer  in 
which  the  corpuscles  lie:  their  finer  subdivisions  extend 
obliquely  into  the  molecular  layer.  The  inwardly  directed 
apex  of  the  corpuscle  gives  rise  to  a  stout  axis-cylinder 
process  which  penetrates  the  layer  next  within  and  there 
bends  abruptly  to  run  backward  in  that  layer,  giving  off 
collaterals  whose  terminal  ramifications  are  in  the  mole- 
cular layer,  and  eventually  to  become  the  axis-cylinder  of 
a  medullated  fibre  of  the  olfactory  tract. 

The  form  and  relations  of  the  mitral  corpuscles  are  sub- 
ject in  different  vertebrates  to  variations  in  detail  that  are 
of  such  importance,  as  bearing  upon  their  functions,  as  to 
merit  description  here.  Tn  mammals  generally  each  mitral 
corpuscle  bears  but  a  single  descending  process:  this  may, 
however,  divide  and  send  branches  to  more  than  one  glom- 
erulus. In  birds  each  mitral  corpuscle  gives  off  several 
descending  processes  to  as  many  glomeruli :  in  either  of 
these  two  ways  a  single  corpuscle  is  put  in  relation  with 
a  number  of  the  bipolar  nervous  elements  of  the  olfactory 
mucosa.  In  some  mammals,  however,  the  glomeruli  are 
relatively  large,  and  each  receives  the  dendritic  ramifica- 
tions of  several  descending  processes  from  as  many  mitral 
corpuscles:  in  such  cases  a  single  bipolar  element  may 
transmit  a  stimulus  to  several  mitral  corpuscles:  this  lat- 
ter condition  obtains  in  the  olfactory  bulbs  of  mammals 
possessed  of  a  high  degree  of  olfactory  sensibility. 

Within  the  layer  of  mitral  corpuscles  is  found  the  granu- 
lar layer  or  deep  layer  of  fibres:  both  of  these  terms  being 


ciiMTKu  xxiii      Tin:  ouc.ANS  OF  siMXiAL  shnsp:.    829 

ai);)lic(l  to  the  tilth  stratum  as  here  defined. ,  Omitting 
tro  11  eoiisideration  tlie  ependymal  liniiiij;  of  the  eavity  of 
the  bulb,  by  some  regarded  as  belonging  to  this  layer,  but 
which  does  not  differ  in  any  essential  respect  from  the 
ependvma  which  everywhere  lines  the  cavities  of  the  ner- 
vous axis,  the  stratum  consists  almost  exclusively  of  the 
two  kinds  of  nervous  elements  indicated  by  the  titles 
above  given.  The  principal  constituents  are  the  nervoUS 
fibres,  which  represent  the  axis-cylinder  processes  already 
described  as  entering  this  layer  from  the  subjacent  corpus- 
cles:  as  these  pass  along  the  length  of  the  bulb  they  give 
off  numerous  collaterals,  of  which  some  run  horizontallv 
to  end  among  the  adjacent  granules;  others  descend  ver- 
tically to  terminate  by  interlacing  ramifications  among 
the  lateral  protoplasmic  processes  of  the  mitral  corpus- 
cles. 

The  name  of  granule  corpuscles  is  used  to  designate 
the  abundant  cellular  elements,  probably  nervous  in  char- 
acter, which  are  grouped  in  numerous  clusters  through- 
out the  laver,  the  fi.bres  above  mentioned  running  in  inter- 
lacing  bundles  among  these  clusters.  They  vary  more  or 
less  in  form,  but  are  usually  provided  at  their  inner  ex- 
tremities with  several  short,  slender,  rapidly  branching 
processes:  the  outer  extremity  bears  a  single  stouter 
process  which  runs  to  the  inner  surface  of  the  molecular 
layer,  there  to  ramify  among  the  lateral  processes  of  the 
mitral  corpuscles.  Other  corpuscular  elements,  which  are 
more  doubtfully  nervous  in  function,  have  been  described 
in  this  layer. 

The  medullated  fibres  of  this  layer  pass  through  the  ol- 
factory tract  to  enter  the  hemisphere  and  there  to  be  dis- 
tributed to  their  destinations  in  the  cortex.  In  addition, 
Cajal  describes  in  the  tract  eflferent  fibres  which  pass  into 
the  deep  fibre  layer  of  the  bulb  to  end  in  arborizations 
which  are  situated  chiefly  among  the  central  processes  of 


33U  PART    II.      HISTOLOGICAL   ANATOMY. 

the  granule  corpuscles.  In  those  mammals  in  which  the 
cavity  of  the  bulb  is  ol)literated  in  the  adult,  the  ependy- 
mal  lining  is  replaced  b\^  a  gelatinous  mass  containing 
numerous  neuroglia  cells.  The  dorsal  portion  of  the 
bulb  is  in  mammals  generalU'  far  simpler  in  structure 
than  the  ventral,  the  latter,  by  virtue  of  its  position,  being 
brought  into  far  more  intimate  relations  with  the  olfac- 
tory mucosa. 

At  the  close  of  the  description  of  the  structure  of  the 
taste-buds  reference  u^as  made  to  the  close  relationship 
of  the  senses  of  taste  and  of  smell,  and  a  possible  resem- 
blance in  structure  in  the  two  organs  was  intimated. 
That  resemblance,  if  it  exists,  is  chiefly  between  the  gus- 
tatory and  the  olfactory  cells :  the  former  are  strikingly 
like  the  latter  as  regards  their  general  form,  and  particu- 
larly as  regards  the  central  nucleated  portion  and  the 
peripheral  process.  As  we  pass  inward,  however,  the 
likeness  is  less  evident :  the  olfactory  cell  is  plainly  a  ner- 
vous element,  being  continued  by  an  axis-cylinder  process 
which  ends  by  arborizations  in  one  of  the  glomeruli  of  the 
olfactory  bulb:  if  the  gustatory  cell  is  a  nervous  element, 
its  inner  portion  must  be  regarded  as  a  corresponding 
arborization-region,  very  greatly  reduced,  and  probably 
discharging  the  impulses  which  it  transmits  upon  the 
intrabulbar  ramifications  of  the  fibres  of  the  glossopharyn- 
geal nerve.  Such  an  arrangement,  if  it  exists,  is  without 
parallel  as  far  as  known ;  but  would  find  its  nearest  repre- 
sentation in  the  olfactory  apparatus.  If,  on  the  other 
hand,  the  gustatory  cells  are  epithelial,  and  not  nervous, 
the  structure  of  the  taste-bud  approaches  most  nearly  to 
that  of  a  tactile  or  pressure-organ,  a  form  of  sensation 
having  little  relation  to  the  sense  of  taste,  which  resem- 
bles that  of  smell  (and  no  other  sense)  in  that  it  enables 
us  to  take  cognizance  of  stimuli  that  must  be  regarded  as 


CHAPTER    XXIII.      1  HI.   ORGANS  OF  SPECIAL  SENSK.     3^1 

essentially  chemical.  It  should  be  noted  that  true  gusta- 
tory sensations  are  also  received  on  portions  of  the 
tongue  in  which  taste-buds  have  not  been  found,  and  that 
these  organs  have  been  descriljed  upon  surfaces  other 
than  that  of  the  tongue  which  are  certainly  not  gusta- 
tory. We  are  here  undoubtedly  confronted  with  a  prob- 
lem whose  solution  depends  upon  discoveries  yet  to  be 
made. 


The  apparatus  of  sight  is  far  more  complex  in  structure 
than  that  of  smell,  alike  in  its  essential  portions  and  in 
those  which  are  accessory  thereto.  The  former  include 
the  capsule,  fibrous  in  man  and  in  the  mammals  generalh' 
throughout  the  larger  portion  of  its  wall  (though  ]iartl\- 
cartilaginous  or  bony  in  some  vertebrates) ;  the  apparatus 
of  refraction,  with  its  mechanisms  of  adjustment;  and 
the  structures  directly  involved  in  the  reception  of  light 
stimuli  and  their  conversion  into  nervous  impulses.  The 
accessory  parts  are  the  protecting  eyelids;  the  investing 
membrane  common  to  them  and  to  the  eyeball ;  and  the 
glands  whose  secretions  maintain  the  proper  condition  of 
this  membrane.  It  will  be  convenient  to  proceed,  in  des- 
cription, from  the  more  external  accessory  parts  to  the 
more  deeply  seated  and  more  complex  essential  structures: 
in  each  case  considering  anatomical  characters  only  in  so 
far  as  necessary  for  the  elucidation  of  the  histology  of  the 
parts  in  question. 

The  eyelids  are  essentially  muscular  folds  of  the  skin, 
modified  chiefly  upon  their  inner  surfaces.  The  outer  sur- 
face resembles  the  skin  of  adjacent  portions  of  the  face  in 


332  PART    II.      HISTOLOGICAL    ANATOMY. 

the  presence  of  diminutive  hairs,  accompanied  by  small 
sebaceous  glands,  and  by  occasional  sweat  glands :  it  is 
thrown  into  small  irregular  folds.  The  underlying  corium 
is  loose  in  texture:  it  also  differs  from  that  of  the  rest  of 
the  skin  of  the  face  in  the  presence  of  considerable  numbers 
of  branched  pigment  corpuscles.  At  the  free  margin  ol  the 
lid  the  surface  curves  inward ;  the  corium  becomes  more 
dense;  and  the  hair  follicles  are  suddenly  enkirged  for  the 
development  of  the  long,  stout,  and  recurved  cilia,  or  eye- 
lashes. Sebaceous  glands  open  into  the  follicles  of  the 
cilia,  as  do  also  some  of  the  ducts  cf  modiiied  sweat 
glands  known  as  the  glands  of  Moll,  others  opening 
freely  at  the  surface. 

As  the  integument  approaches  the  inner  surface  of  the 
lid  it  bends  almost  at  a  right  angle  at  the  palpebral  bor- 
der, and  at  the  same  time  becomes  modified  in  "structure 
to  form  the  palpebral  conjunctiva,  which  lines  the  sur- 
face of  the  lid  in  contact  with  the  eyeball.  Beneath  the  in- 
tegument is  situated  the  orbicularis  muscle,  composed  of 
striated  fibres  whose  bundles  run  in  a  general  way  par- 
allel to  the  palpebral  border:  the  group  of  bundles  situ- 
ated just  within  the  border  fold,  and  separated  from  the 
mass  of  the  orbicularis  by  the  follicles  of  the  cilia,  is  dis- 
tinguished as  the  ciliary  or  marginal  muscle,  or  as  the 
muscle  of  Riolan.  Just  interior  to  the  orbicularis  mus- 
cle lies  the  palpebral  fascia,  a  layer  of  fibrous  tissue  which 
separates  the  tegumental  from  the  conjunctival  portion 
of  the  lid:  in  the  upper  lid  it  is  blended  with  the  tendon  of 
the  levator  muscle. 

The  palpebral  conjunctiva  consists  of  a  layer  of  strati- 
fied columnar  epithelium  containing  scattered  goblet  cells, 
and  resting  upon  a  definite  basement  membrane:  and  a 
dense  mass  of  fibrous  tissue,  the  tarsus,  or  the  tarsal  carti- 
lage (erroneously  so-called,  as  it  is  entirelv  devoid  of  carti- 
lage corpuscles).  The  stratified  columnar  epithelium  of  the 


CHAI'TKK    XXIII.    THE  ORGANS  OF  SPKCIAL  SENSE.      333 

conjinictiva  passes  tjradually  at  the  palpebral  border  into 
the  stratified  squamous  epithelium  ot  the  integument : 
the  tarsus  may  be  regarded  as  the  continuation  of  the 
denser  portion  of  the  tegumentary  corium.  Upon  the 
inner  surface  of  the  lid  there  can  be  seen  with  the  naked 
eye.  a  number  ol  vertical  rows  o(  apparently  granular 
masses,  of  a  yellowish  color.  These  are  the  tarsal  or 
Meibomian  glands.  com])ound  structures  of  the  sebace- 
ous type  which  are  imbedded  in  the  tarsus:  each  consists  of 
a  straight  or  somewhat  curved  conducting  tube  or  duct 
lined  with  cuboidal  epithelium,  into  the  sides  of  which 
o])en  numerous  sebaceous  saccules  resembling  in  everv  es- 
sential those  found  in  connection  with  the  hair  follicles; 
the  ducts  open  by  minute  orifices  upon  the  margin  of  the 
lid.  their  mouths  being  lined  for  a  short  distance  with 
stratified  squamous  e])ithelium. 

Along  the  proximal  margin  of  the  tarsus,  and  partly 
imbedded  therein  are  scattered  branched  tubular  glands  of 
the  serous  type,  the  accessory  tear  glands:  they  discharge 
their  secretion  upon  the  adjacent  conjunctival  surface. 
The  conjunctival  surface  of  this  vicinity  is  frequentlv 
thrown  into  folds,  chiefly  involving  the  epithelium,  whose 
appearance  in  cross  section  has  led  to  their  being  de- 
scribed as  glands.  The  connective  tissue  between  the  base- 
ment membrane  of  the  conjunctiva  and  the  tarsal  plate 
contains  diffuse  adenoid  tissue  which  is  occasionallvgath- 
ered  into  nodules  in  the  human  subject:  in  some  of  the 
lower  mammals  these  nodules  are  quite  numerous  and 
well-defined. 

Bevond  the  base  of  the  eyelids  the  conjunctiva  passes 
over  upon  the  eyeball  at  the  fornix  conjunctivae.  Goblet 
cells  are  more  numerous  here  than  upon  the  palpebral  sur- 
face; the  fibrous  portion  contains  a  number  of  distinct 
adenoid  nodules,  and  a  few  mucous  glands :  inwardlv.it 
passes  over  into  a  loose  la\er  of  subconjunctival   areolar 


334  PART   II.      HISTOLOGICAL  ANATOMY. 

tissue  which  permits  of  a  considerable  amount  of  motion. 
The  continuation  of  the  conjunctiva  upon  the  e\'eball  will 
be  best  described  in  connection  with  that  structure.  The 
plica  semilunaris,  a  vertical  curved  fold  at  the  inner 
angle  of  the  e3'-e,  representing  the  third  e\^elid  of  many 
lower  vertebrates,  is  a  mere  fold  of  conjunctiva;  it  con- 
tains internall}'  in  some  mammals,  and  sometimes  in  man, 
a  thin  slip  of  h3'aline  cartilage:  as  well  as  a  rudimentary 
racemose  gland  regarded  as  representing  the  Harderian 
gland  generalh'^  present  in  the  eyes  of  those  vertebrates 
which  have  a  functional  third  eyelid.  The  adjacent  ca- 
runcle is  a  rounded  fatty  mass,  with  an  investment  agree- 
ing with  the  integument  in  structure  and  containing  mi- 
nute hairs  and  modified  sweat  glands.  The  conjunctiva, 
w^hich  is  highH'  sensitive,  is  richly  supplied  with  the  nerve 
terminals  already  described  as  end-bulbs. 

The  lachrymal  gland,  situated  in  the  supero-lateral 
portion  of  the  orbit,  consists  of  two  somewhat  distinct 
portions,  sometimes  described  as  the  superior  and  inferior 
lachrymal  glands.  The  whole  mass  consists  of  an  aggre- 
gation of  compound  racemose  glands  which  open  by  in- 
dependent ducts  upon  the  conjunctival  surface  in  the  region 
of  the  superior  fornix.  The  acini,  which  may  be  either 
simple  or  branched,  are  lined  by  granular  cells  with  large 
spherical  nuclei,  agreeing  in  this  respect  with  the  alveoli 
of  serous  glands ;  from  which  the}'  differ,  however,  b}^  the 
presence  in  each  of  a  distinct  and  sometimes  a  large 
lumen.  They  open  into  ductules  lined  by  flattened  or  low 
columnar  cells:  these  lead  into  ducts  whose  epithelium  is 
distinctly  columnar,  and  in  which  a  second  layer  of  small 
cells  has  been  described  as  situated  near  the  basement 
membrane. 

The  secretion  of  the  lachrjanal  gland,  after  washing  the 
surface  of  the  eyeball,  is  carried  away  by  the  lachrymal 


C!I.M'TI-:K  XXIII.     TIIK  ORC/ANS  OF  SI'KCIAL  SENSE.       335 

canals,  whlcli  open  on  the  ])alj)chral  borders  near  their 
inner  extremities.  Each  canal  is  lined  with  stratified  squa- 
inons  epilheliuin,  which  rests  upon  a  tihrous  layer  rich  in 
clastic  fibres:  external  to  this  is  a  layer  of  striated  mus- 
cular fibres  which  are  <2^cnerally  dis|)osed  lon<;itudinally. 
The  canals  discharge  into  the  lachrymal  sac,  which  is 
continued  to  the  nasal  cavity  by  the  nasal  or  lachrymal 
duct.  The  sac  and  the  duct  are  both  comj^osed  of  ahi  elas- 
tic fibrous  layer  containing  considerable  adenoid  tissue, 
and  lined  bv  a  mucous  membrane  which  is  invested  by 
columnar  ejiithelium  resembling  that  of  the  nasal  cavity. 

The  visual  capsule  consists  of  two  distinct  strata ;  the 
outer,  or  skeletal,  which  is  known  throughout  the  greater 
]K)rtion  of  the  eyeball  as  the  sclerotic,  but  is  transformed 
in  front  to  form  the  transparent  cornea;  and  the  inner,  or 
musculo-vascular,  which  is  composed  of  the  posterior 
choroid,  and  the  anterior  iris:  the  essential  nervous  struc- 
ture of  the  eye  lies  immediately  interior  to  the  choroid. 
The  cornea  is  a  part  of  the  refracting  apparatus  of  the 
eve,  and  the  iris  a  portion  of  the  regulatory  mechanism  : 
but  each  ma\'  be  conveniently  described  in  connection  with 
the  stratum  of  which  it  is  a  portion. 

The  whitish  sclerotic  is  a  dense  fibrous  layer  resembling 
somewhat  a  greatly  thickened  membrane;  the  interlacing 
fibre  bundles  are  arranged  chiefly  in  antero-posterior  and 
in  transverse  directions :  elastic  fibres  are  sparingly  pres- 
ent:  the  fixed  corpuscles  are  flattened,  and  lie  in  definite 
lacunae  of  irregular  form.  The  inner  kiyer  is  rich  in  brown- 
ish pigment  and  is  known  as  the  lamina  fusca:  between 
it  and  the  outer  layer  of  the  choroid  are  extensive  lymjih 
spaces  lined  with  endothelium  and  traversed  by  blood- 
vessels and  strands  of  connective  tissue.  The  sclerotic  is 
nearh'  twice  as  thick  in  its  posterior  as  in  its  anterior  por- 
tion :   where  the  optic  nerve  enters  the  eye  the  sclerotic  be- 


336  PART   II.      HISTOLOGICAL   ANATOMY. 

comes  continuous  with  thesheath  of  that  cylindrical  bodv: 
the  circular  area  enclosed  is  suddenly  thinned,  and  is 
pierced  by  a  number  of  small  openings;  it  is  therefore  des- 
ignated the  lamina  cribrosa.  Over  the  larger  portion  of 
the  sclerotic  its  outer  surface  is  invested  by  a  thin  layer  of 
connective  tissue  loosely  uniting  it  to  the  capsule  of  Tenon, 
a  membranous  sac  lined  with  endothelium  and  enclosing 
the  space  of  Tenon  by  means  of  which  the  e^'eball  is  sep- 
arated from  the  fat  masses  lining  the  orbit.  In  front  the 
sclerotic,  is  invested,  as  far  as  the  scleral  sulcus  bv  which 
it  is  separated  from  the  cornea,  by  the  scleral  conjunctiva: 
this  consists  chiefly  of  stratified  squamous  epithelium 
resting  upon  a  thin  fibrous  membrane  which  is  connected 
to  the  sclerotic  by  a  scanty  layer  of  looser  connective  tis- 
sue: it  contains  numerous  end-bulbs,  and,  as  is  well  known, 
is  extremely  sensitive. 

The  cornea  is  readily  distinguishable  from  the  sclerotic, 
not  only  by  its  transparency,  but  also  by  its  greater  con- 
vexity. Its  outer  surface  is  covered  by  a  layer  of  stratified 
squamous  epithelium  continuous  with  that  ol  the  scleral 
conjunctiva:  the  layer  is  several  cells  deep,  the  outer  ele- 
ments being  strongly  flattened,  but  retaining  their  nuclei, 
and  the  inner  being  digitated  in  a  manner  similar  to  the 
prickle-cells  of  the  epidermis.  The  deepest  cells  rest  on  a 
thin,  dense,  homogeneous  layer  of  closely  felted  fibres,  the 
membrane  of  Bowman,  or  external  limiting  membrane, 
which  possibly  represents  the  fibrous  portion  of  the  con- 
junctiva. Beneath  this  membrane,  and  closel}^  connected 
with  it  is  the  substantia  propria  of  the  cornea,  a  mass  of 
the  corneal  tissue  described  in  detail  in  a  previous  chapter: 
at  its  margin  it  passes  over  into  the  substance  of  the  scle- 
rotic, of  which  it  is  presumably  a  modification.  Internally 
this  mass  is  invested  with  a  thin  homogeneous  elastic  in- 
ternal limiting  membrane,  otherwise  known  as  the 
membrane    of    Descemet:    its    inner  surface  is  covered 


CHAPTER    XXIII.     THK  OKOANS  OF  SPECIAL  SENSK.      337 

with  a  layer  of  endoLhcliuni  and  bounds  the  anterior 
chamber  of  the  eye.  Around  its  margin  the  meml)rane  of 
Deseeniet  is  continued  by  a  number  of  processes  to  form 
the  pectinate  ligament  by  which  the  cornea  is  attached 
to  the  iris. 

The  choroid  is  nearly  coextensive  with  the  sclerotic.  It 
consists  of  a  vascular  layer  of  fibrous  tissue  which  is  ex- 
ceedingly rich  in  large  pigment  corpuscles,  imparting  to  it 
a  color  which  varies  from  brown  to  black:  in  the  human 
eye  it  is  dark  brown  in  color.  Its  outermost  portion,  con- 
sisting entirely  of  pigmented  connective  tissue,  forms  a 
layer  distinguished  as  the  lamina  suprachoroidea :  it 
lies  immediate!}'  within  the  lamina  fusca  of  the  sclerotic, 
from  which  it  is  largely  separated,  as  has  been  already 
stated,  by  extensive  lymph  spaces  lined  with  endothelium. 
The  body  of  the  choroid  is  rich  in  bloodvessels,  which  are 
disposed  in  tw^o  strata;  an  outer  containing  the  arteries 
and  veins,  wdiich  are  arranged  in  a  characteristic  manner, 
and  an  inner,  the  capillary  tunic,  or  tunic  of  Ruysch:  the 
fibrous  structure  between  the  two  is  a  layer  of  connective 
tissue  rich  in  elastic  fibres  which  in  some  mammals  is  so 
well  developed  as  to  form  a  distinct  layer  which  is  visible 
through  the  capillary  tunic  and  the  retina,  and  is  known 
as  the  tapetum.  Within  the  capillary  tunic  is  a  thin  trans- 
jjarent  layer,  the  membrane  of  Bruch,  or  vitreous  mem- 
brane. 

The  anterior  portion  of  the  choroid  is  modified  by  the 
foldings  of  its  inner  surface  known  as  the  ciliary  processes, 
and  by  the  thickening  due  to  the  presence  of  the  layer  of 
bundles  of  smooth  muscular  fibres  termed  the  ciliary 
muscle:  the  whole  region,  including  a  marginal  zone,  the 
ciliary  ring,  in  which  the  capillary  tunic  is  less  well  de- 
veloped than  in  the  choroid  generally,  is  sometimes  desig- 
nated the  ciliary  body.  The  ciliary  processes,  upwards 
of  seventy  in    number  in  the  human  eye,  are  meridion- 


33H  PART   II.      HISTOLOGICAL   ANATOMY. 

ally  disposed  folds  which  begin  just  anterior  to  the  cili- 
ary ritig  and  rise  gradually  to  the  height  of  a  half  of  a 
millimetre  or  so,  to  terminate  abruptly  at  the  margin  of 
the  iris:  like  the  rest  of  the  choroid,  they  are  quite  vascu- 
lar, the  vessels  being  imbedded  in  a  pigmented  stroma  of 
connective  tissue,  and  are  limited  internally  by  the  vitreous 
membrane.  Upon  their  surfaces  are  pouch-like  depressions 
lined  by  the  epithelial  layer,  presently  to  be  described, 
with  which  the  vitreous  membrane  is  invested  in  this  re- 
gion;  these  have  been  called  ciliary  glands :  their  gland- 
ular function  is,  however,  doubtful. 

The  ciliary  muscle  is  b_v  some  regarded  as  a  portion  of 
the  choroid,  by  others  as  interposed  between  it  and  the 
sclerotic.  It  is  composed  of  bundles  of  smooth  muscular 
fibres  (of  striated  fibres  in  birds),  most  of  which  arise 
from  the  pectinate  ligament  at  the  region  where  the  scle- 
rotic, the  cornea,  and  the  iris  come  together:  of  these  the 
greater  number  run  meridionally  to  be  inserted  into  the 
choroid,  and  are  therefore  sometimes  regarded  as  forming 
a  distinct  muscle,  the  tensor  choroideae:  the  remainder, 
known  as  radial  bundles,  run  obliqueh'  to  those  just  de- 
scribed, assuming  a  direction  which  tends  toward  the  cen- 
tre of  the  eye,  and  being  inserted  in  the  ciliary  processes. 
Other  bundles,  internally  situated,  are  arranged  in  a  more 
or  less  definite  circular  tract,  known  as  the  ring-muscle 
of  Mueller-  The  ciliary  muscle  as  a  whole  is  triangular  in 
cross  section :  it  is  thickest  in  hypermetropic  eyes,  due 
largely  to  an  increase  in  the  size  of  the  ring-muscle. 

The  stroma  of  the  choroid  is  continued  forward  from  be- 
yond the  ciliary  body  to  form  the  principal  portion  of  the 
iris:  the  latterishighly  vascular,  but  not  so  much  so  as  the 
rest  of  the  musculo-vascular  layer:  it  is  also  somewhat  dif- 
ferent in  texture,  approaching  more  nearh'tothe  vStructure 
of  retiform  tissue.  It  contains  numerous  pigment  cor- 
puscles, of  different  colors  in  different  persons,  on  whose 


CIIAPTHK    XXIII.      Tin:  ORGANS  OF  SIM:CIAI-  SKXSK.      331) 

presence  the  color  of  the  iris  depends:  i\n  exception  to  this 
occurs  in  cases  where  the  iris  is  blue:  here  pigment  cor- 
puscles are  wanting  in  the  stroma,  the  color  depending 
entirely  on  the  appearance  of  the  post-iridal  pigment  (to 
be  described-later)  as  seen  through  the  body  of  the  iris. 

The  anterior  surface  of  the  stroma  is  somewhat  con- 
densed, and  leaves  a  layer  of  endothelial  corpuscles  con- 
tinuous at  the  irido-corneal  angle  with  those  w'hich  invest 
the  membrane  of  Descemet.  The  posterior  surface  is 
formed  by  a  homogenous  layer  continuous  with  the  vitre- 
ous membrane  of  the  choroid  and  the  ciliary  body :  against 
it  lies  the  layer  of  pigment  corpuscles  above  referred  to. 
Imbedded  in  the  stroma  of  the  iris  near  the  pupillary  mar- 
gin is  an  angular  layer  of  smooth  muscular  fibres,  the 
sphincter  papillae:  near  the  posterior  surface  are  radi- 
iiting  bundles  forming  a  thin  la3'er  which  is  not  continu- 
ous, known  as  the  dilator  pupillae:  its  existence  is  ques- 
tioned by  some  histologists,  the  demonstration  of  the 
scattered  bundles  of  smooth  muscular  fibres,  as  distin- 
guished from  the  adjacent  bloodvessels  and  bundles  of 
connective  tissue,  being  quite  difficult. 

The  region  lying  between  the  outer  margin  of  the 
iris  and  the  sclero-corneal  sulcus  is  one  of  great  import- 
ance; the  iris,  the  choroid,  the  ciliarx' muscle,  the  cornea 
and  the  sclera  all  coming  together  in  this  vicinit\'.  Just 
external  to  the  irido- corneal  angle,  and  among  the 
fasciculi  which  make  up  the  pectinate  ligament,  lies  a  loose 
network  of  trabeculae  of  white  and  elastic  fibres  whose 
interstitial  cavities,  imperfectl}^  lined  with  endothelial  cells, 
are  known  as  the  spaces  of  Fontana:  they  communicate 
freeK'  with  the  anterior  chamber  of  the  eye  and  contain 
the  same  fluid.  External  to  these  and  fairly  within  the 
sclerotic  portion  of  the  region  is  situated  an  annular 
space,  irregularly  flattened  and  in  places  subdivided :  it 
is  called  the  canal  of  Schlemm  :  whether  it  is  a  lymphatic 


340  PART   II.      HISTOLOGICAL   ANATOMY. 

or  a  venous  channel,  and  whether  or  not  it  communicates 
with  the  spaces  of  Fontana  are  still  matters  of  dispute. 

The  parts  concerned  with  the  processes  of  refraction  by 
means  of  which  distinct  images  of  things  seen  are  formed 
on  the  sensitive  surface  within  the  eye  are  the  cornea,  a 
description  of  which  has  already  been  given;  the  aqueous 
humor,  a  water}^  fluid  in  which  leucocytes  are  occasionalh^ 
found,  but  containing  no  other  tissue  elements,  which  fills 
the  space  between  the  cornea  and  the  capsule  of  the 
lens:  the  crystalline  lens,  with  its  capsule,  by  means  of 
which  it  is  suspended  at  right  angles  to  the  eye  immedi- 
ately behind  the  iris  ;  and  the  vitreous  humor,  which  fills 
the  cavity  posterior  to  the  lens.  The  regulatory  mechan- 
isms are  the  iris,  which,  by  modif\ang  the  size  of  the  pupil- 
lary aperture,  governs  the  amount  of  light  which  passes 
through  the  lens  and  indirectly  (to  some  extent)  the  sharp- 
ness of  the  image  formed  by  it ;  and  the  ciliary  muscle, 
whose  action  modifies  the  convexity  of  the  lens  and  thus 
affects  its  definition :  an  account  of  these  having  already 
been  given,  there  remain  for  description  the  lens  and  the 
vitreous  humor,  with  the  capsules  by  which  they  are 
surrounded. 

The  crystalline  lens  is  composed  of  an  epithelial  layer 
and  a  fibrous  mass  whose  components  are  modified  epi- 
thelial corpuscles.  The  epithelial  layer  consists  of  cu- 
boidal  elements  which  form  nearly  the  whole  of  the  ante- 
rior surface:  as  they  approach  the  equator  they  become 
columnar,  and  as  thej^  reach  that  region  become  greatly 
elongated  and  assume  the  shape  of  the  long  hexagonal 
])rismatic  fibres  of  which  the  greater  part  of  the  lens  is 
composed :  at  the  equator  these  retain  their  nuclei  for 
some  time  after  birth,  but  those  of  the  greater  portion  of 
the  mass  are  quickly  lost. 

The  fibres  run  from  the  anterior  to  the  posterior  por- 


CHAPTER    XXIII.     IHK   ORGANS  OF  SPECIAL  SKXSE.      341 

tion  of  the  mass,  l)elii<.^  so  disposed  that  their  extremities 
eome  to<2^ether  in  eaeh  region  along  radifiting  sutural 
pL'ines  ])rimarily  three  in  number;  the  sutures  of  the  an- 
terior region  alternate  with  those  of  the  posterior  region 
in  position;  as  a  consequence,  the  fibres  pass  obliquely 
from  one  region  to  another:  the  degree  of  obliquity  is  in- 
creased by  the  fact  that  the  length  of  the  fibres  is  such 
that  those  which  arise  nearest  the  centre  of  the  anterior 
region  terminate  posteriorly  nearest  the  equator;  and 
vice  versa.  As  a  consequence  of  this  arrangement  of  the 
fibres,  stellate  figures  are  formed  w^hich  can  be  easil}'  seen 
in  the  artificially  hardened  lens,  and  traces  of  which  have 
been  discovered  in  the  living  e\^e  by  the  aid  of  the  ophthal- 
moscope. Hardening  in  alcohol  reveals  a  tendency  to 
lamination,  particularly  in  the  outer  portion,  the  laminae 
peeling  ofifin  triangular  patches  which  separate  along  the 
sutural  planes. 

The  capsule  of  the  lens  is  a  transparent  elastic  sac 
which  completely  encloses  that  body.  It  is  apparenth' 
homogenous  in  structure  in  the  adult,  but  is  regarded  b}- 
some  as  composed  of  two  laminae:  an  inner,  cuticular  in 
character,  formed  b_v  the  activity  of  the  epithelial  elements 
within  while  still  embryonic :  and  an  outer,  composed  of 
fibrous  tissue.  It  is  thicker  in  front  than  behind,  in  rela- 
tion with  the  greater  change  of  curvature  which  takes 
place  in  the  more  highly  convex  posterior  surface  in  visual 
adjustments.  The  support  of  the  capsule  and  lens  in  place 
will  be  described  later. 

The  vitreous  humor,  or  vitreous  body  (as  it  also  called), 
is  a  semi-fluid  mass  of  extreme  tranparenc}'  derived  from 
the  modification  of  a  quantity'  of  gelatinous  tissue  b\'  the 
infiltration  of  lymph  to  such  an  extent  that  over  ninety- 
eight  per  cent,  of  its  substance  is  water.  Slender  trans- 
parent fibres  are  scattered  through  the  mass,  and  occa- 
sional corpuscles  are  found  in  it :  these  are  generally  of  ex- 


342  PART   ir.      HISTOLOGICAL   ANATOMY.  ' 

qeedingly  irregular  shape,  and  are  often  extensively  vacuo- 
lated ;  they  are  probably  modified  leucocytes.  Under  cer- 
tain methods  of  hardening  the  vitreous  body  can  be  seen 
to  present  evidences  of  a  laminar  structure;  whether 
this  is  real,  or  the  result  of  the  treatment  employed,  is  not 
3^et  certain. 

The  vitreous  body  is  invested  by  a  thin  transparent 
capsule,  the  hyaloid  membrane,  which  is  structureless 
throughout  the  greater  portion  of  its  extent.  Opposite 
the  optic  nerve  it  is  reflected  forward  to  line  the  slender 
hyaloid,  canal  which  perforates  the  vitreous  body,  termi- 
nating in  front  at  a  point  opposite  the  centre  of  the  pos- 
terior surface  of  the  crystalline  lens:  here  the  hyaloid 
membrane  is  reflected  outward  from  the  anterior  extremi- 
ty of  the  canal  to  line  the  patellary  fossa,  on  the  anterior 
surface  of  the  vitreous  body,  which  receives  the  convexity 
of  the  lens  and  its  capsule,  the  latter  being  in  contact 
with  the  hyaloid  membrane. 

At  the  margin  of  the  patellary  fossa  the  hyaloid  mem 
brane  lining  it  becomes  continuous  with  that  investing 
the  outer  surface  of  the  vitreous  body:  here  the  membrane 
is  distinctly  fibrous,  the  fibres  in  some  cases  penetrating 
the  gelatinous  mass  within.  It  gives  off  from  its  outer 
surface  a  fibrous  layer  which  closely  invests  the  ciliary 
processes  as  the  zone  of  Zinn,  or  zonula  ciliaris :  its  free 
portion  extends  beyond  the  ciliary  processes  to  be  inserted 
upon  the  capsule  of  the  lens  at  its  equator,  thus  forming 
the  suspensory  ligament  of  the  lens  above  mentioned. 
The  mode  of  insertion  of  the  suspensory  ligament  is  such 
as  to  leave  a  narrow  circular  space  between  the  two 
layers  of  the  hyaloid,  known  as  the  canal  of  Pettit. 

The  structures  directly  involved  in  the  reception  of  light 
stimuli  and  their  conversion  into  nervous  impulses  are 
contained  in  the  retina,  a  highly  complex  organ  formed  by 


CHAPTKR  XXIII.    THE  OK(iANS  OF  SPECIAL  SENSE.      34-3 

the  modification  of  a  direct  outgrowth  from  the  brain. 
This  outgrowtli,  at  first  vcsicuhir  in  form,  is  afterwards 
doubled  upon  itself  in  such  a  manner  as  to  form  a  sphe- 
roidal cup  composed  of  two  layers  and  situated  chiefl\' 
between  the  vitreous  humor  and  the  choroid,  its  basal 
stalk-like  portion  beconiinfr  the  optic  nerve.  The  fibres  of 
the  optic  nerve  are  continued  over  the  inner  surface  ot  the 
retina,  in  a  manner  to  be  presently  described,  as  far  as  the 
outer  or  posterior  extremities  of  the  ciliary  processes;  the 
sinuous  line  which  marks  the  limit  of  their  distribution 
being  known  as  the  ora  serrata.  In  front  of  this  line  the 
surface  of  the  ciliary  processes  is  invested  by  a  much  sim- 
pler ciliary  portion  of  the  retina,  which  is  in  turn  con- 
tinued into  the  iridal  portion,  or  uvea,  which  lines  the  in- 
ner surface  of  the  iris:  the  structure  of  these  outlying  por- 
tions will  be  best  understood  after  a  description  of  that 
of  the  principal  portion  of  the  retina. 

When  examined  by  the  ordinary  methods  of  hardening 
and  staining  the  retina  shows  in  transvervse  section  a  num- 
ber of  distinct  strata  or  layers,  tolerabl}^  uniform  in 
structure  throughout  the  greater  portion  of  its  extent. 
These  are  usually  stated  as  eight  in  number:  here,  how- 
ever, as  in  the  other  sense  organs,  our  views  have  in  re- 
cent 3'ears  been  greatly  modified  by  the  results  of  the 
chromate  and  silver  method.  An  account  of  the  eight 
layers  will  first  be  given,  and  the  relation  of  their  compo- 
nents as  now  understood  subsequently  considered. 

Beginning  at  the  inner  or  anterior  surface  of  the  retina, 
there  may  be  discerned  next  to  the  hyaloid  membrane 
which  invests  the  vitreous  bod}'^  a  delicate  layer  which  is 
apparently  (and  onK'  apparently)  continuous  :  it  has  been 
designated  the  internal  limiting  membrane:  it  is  in 
reality  a  mosaic  formed  b}^  the  thin  expanded  ends  of  sup- 
porting structural  elements,  the  fibres  of  Mueller,  which 


344  PART    II.      HISTOLOGICAL   ANATOMY. 

pass  verticalh'  toward  (but  not  to)  the  outer  surface  of 
the  retina. 

Next  to  the  internal  limiting  membrane,  so  called,  is 
seen  the  first  definite  stratum  of  the  retina,  the  layer  of 
nerve  fibres.  These,  which  are  non-medullated  in  most 
cases,  are  arranged  in  small  bundles  which  form  a  plexi- 
form  meshwork  over  the  inner  surface,  radiating  from  the 
optic  nerve,  or,  more  exactly,  converging  to  it  from  all 
portions  of  the  retina.  This  layer,  like  nearly  all  the 
others,  is  quite  transparent,  and  is  itself  insensitive  to 
light. 

The  layer  of  nerve  fibres  is  succeeded  outwardly  by  the 
ganglionic  layer,  a  stratum  of  relatively  large  multipolar 
nerve  corpuscles,  either  spheroidal  or  pyriform  in  shape, 
whose  axis-cylinder  processes  are  continuous  with  fibres 
of  the  preceding  layer,  and  whose  other  processes  ramifj- 
in  the  la3^er  next  beyond.  The  ganglionic  layer  varies  in 
thickness  in  different  portions  of  the  retina,  being  in  some 
places  two  or  three  corpuscles  deep,  but  over  the  greater 
portion  of  its  surface  consisting  of  a  single  layer  of  cor- 
puscles: toward  the  ora  serrata  these  become  separated 
from  each  other  by  considerable  intervals. 

Immediately  beyond  the  ganglionic  layer  is  situated  the 
inner  molecular  layer:  this,  which  is  in  most  portions 
of  the  retina  the  thickest  of  the  visible  strata,  is  appar- 
ently composed  of  a  granular  mass,  which  is  in  reality  the 
expression  in  section  of  the  cut  extremities  of  the  rami- 
fying processes  of  the  corpuscular  elements  of  the  layers 
next  adjacent. 

External  to  the  inner  molecular  layer  is  seen  the  inner 
nuclear  layer,  composed  chiefly  of  closely  aggregated 
bipolar  and  multipolar  nerve  corpuscles,  the  former  pre- 
dominating. The  corpuscles  var^^  greatl}^  in  size,  but  are, 
on  the  average,  decidedly  smaller  than  those  of  the  gan- 
glionic layer:   the  disposition  of  their  processes  will  be  de- 


CIIVrTIvK    XXIII.      Tin:  ORC.ANS  OF  SPIXIAL  SKXSE.     34-0 

scribed  later.  Xiicleatcd  cMilari^ciiK'nts  of  the  sustentacu- 
lar  elements,  or  fibres  of  Mueller,  are  also  found  in  this 
Layer  to  some  extent. 

The  inner  nuclear  layer  is  followed  by  the  outer  molecu- 
lar layer,  a  <;ranular  stratum  closely  resembling  in  its 
appearance  the  inner  molecular  layer,  but  differing  from  it 
gi'catlv  in  extent, being  usually  the  thinnest  of  the  various 
la  vers  of  the  retina. 

Bevond  the  outer  molecular  layer,  again,  is  seen  a  layer 
of  cor  j)uscular  elements,  the  so  called  outer  nuclear  layer, 
which  as  ordinarily  seen  resembles  the  inner  nuclear  layer 
as  closely  as  do  the  two  molecular  layers  :  nearly  all  of  the 
constituent  elements  are  distinctly  bipolar,  and,  as  will 
presently  be  shown,  their  relations  are  quite  different  from 
those  of  the  elements  of  the  inner  of  the  two  apparently 
similar  layers.  The  corpuscles  of  this  layer  are,  further- 
more, closely  connected  with  the  elements  of  the  succeed- 
ing layer  in  a  manner  which  differentiates  them  sharply 
from  any  other  retinal  elements. 

The  outer  surface  of  the  outer  nuclear  layer  is  sharply 
defined,  the  sustentacular  tissue  of  the  retina  here  termin- 
ating soabruptly  as  to  lead  to  the  description  of  a  definite 
external  limiting  membrane:  recent  researches  have 
shown  that  the  application  of  the  term  membrane  in  this 
connection  is  even  less  justifiable  than  in  the  case  of  the 
inner  boundary  of  the  retina. 

External  to  the  outer  nuclear  layer,  and  apparently 
resting  upon  the  so-called  external  limiting  membrane,  is 
the  bacillary  layer,  or  layer  of  rods  and  cones.  It  consists 
exclusively  of  the  two  kinds  of  elements  designated  by  the 
latter  of  these  titles.  Those  termed  rods  are  b\^  far  the 
most  numerous  in  nearly  all  portions  of  the  retina :  each 
consists  of  an  inner  or  basal  portion,  somewhat  thicker 
in  the  middle  than  at  the  extremities,  and  an  outer  or 
terminal  portion,  slightl}'  longer  than   the  basal  in   man 


34-6  PART    II.      HISTOLOGICAL   ANATOMY. 

and  most  mammals,  which  is  of  nearly  uniform  diameter 
throughout  its  entire  length.  The  outer  segment  of  each 
rodistransversely  striated,  and  can  be  resolved  into  a  num- 
ber of  thin  disks  by  the  aid  of  certain  reagents :  the  outer 
portion  of  the  basal  segment  is  longitudinally  striated. 
The  cones  resemble  the  rods  in  consisting  of  two  segments: 
the  inner  of  these  is  much  stouter  than  the  basal  segments 
of  the  adjacent  rods,  and  is  thicker  at  the  base  than  at  the 
outer  extremity :  like  the  corresponding  regions  of  the 
rods,  the  outer  portion  of  the  basal  segment  is  longitudi- 
nally striated.  Upon  its  free  extremity  is  seated  the  outer 
segment,  which  is  shorter  than  that  of  the  rods,  and 
tapers  to  a  point :  it  shows  transverse  striations. 

The  layer  of  rods  and  cones  was  for  a  long  time  re- 
garded as  the  limiting  stratum  of  the  retina,  the  adjacent 
layer  of  pigment  cells  being  associated  with  the  choroid, 
with  which  it  is  in  close  contact.  With  advancing  know- 
ledge of  the  embryological  development  of  the  eye,  how- 
ever, it  has  become  evident  that  it  must  be  regarded  as 
retinal  in  nature.  It  consists  of  a  single  stratum  ol  pris- 
matic cells,  hexagonal  in  form  and  so  heavily  loaded  with 
jjigment  as  to  hide  the  large  central  nucleus:  the  outer 
surface  of  the  cell,  in  contact  with  the  choroid,  is  smooth: 
the  inner  is  prolonged  by  numerous  slender  processes 
which  extend  between  the  rods  and  cones  of  the  adjacent 
layer  to  a  distance  which  varies  in  relation  with  the  in- 
tensity of  the  stimulus  acting  upon  the  retina. 

The  layers  of  the  retina,  as  above  described,  are  now 
known  to  be  the  expression  as  seen  under  the  more  familiar 
methods  of  preparation  of  a  system  of  nervous  elements  ar- 
ranged in  a  manner  not  unlike  that  which  is  found  in  the 
other  organs  of  special  sense:  their  disposition  may  be 
briefly  described  as  follows,  disregarding  for  the  present 
the  layer  of  pigment  cells  just  mentioned. 

The  rods  and  the  cones  of  the  bacillary  layer  are  in  real- 


CHAPTER  XXIII.     THE  OnOANS  OF  SPECIAL  SENSE.      34-7 

itv  structurally  continuous  with  the  elements  of  the  outer 
nuclear  laver  in  such  a  manner  that  they  may  with  propri- 
ety be  rci^arded  as  their  peripheral  prolongations,  or  as 
their  greatly  morlified  dendritic  portions.  Each  rod  is 
continued  within  the  external  limiting  membrane  by  a 
slender  filament  of  greater  or  less  extent  which  terminates 
at  the  outer  pole  of  one  of  the  spindle  sha])ed  corpuscles  ot 
the  nuclear  layer:  the  body  of  the  corpuscle  is  transversely 
striated  in  a  characteristic  manner:  from  its  inner  pole  it 
gives  oft'  a  fine  varicose  filament,  the  homologue  of  the 
axis-cylinder  process,  which  extends  to  the  outer  molecu- 
lar layer  and  there  terminates  in  a  small  knob-like  expan- 
sion, representing  a  greatly  modified  terminal  arborization 
the  rod-corpuscles  are  situated  at  various  levels  in  the  outer 
nuclear  layer  from  the  outer  to  the  inner  surface,  their 
peripheral  and  central  filamentous  prolongations  varying 
in  length  in  a  corresponding  manner. 

The  base  of  each  cone  is  continued  beneath  the  external 
limiting  membrane  by  a  stout  strand  of  protoplasm 
which  passes  almost  immediately  into  the  nucleated  cor- 
puscle, the  latter  being  situated  just  within  the  membrane: 
the  corpuscle  is  continued  inward  b\'  a  stout  smooth 
fibre  which  passes  directly  across  the  outer  nuclear  layer 
to  end  within  the  surface  of  the  outer  molecular  layer  by 
a  disk-like  expansion  from  whose  margin  slender  filaments 
are  given  off,  forming  a  rudimentary  arborization. 

The  rod  and  cone  elements,  including  both  the  bacillary 
and  the  nuclear  portions,  as  far  as  the  outer  molecular 
layer,  may  be  regarded  as  forming  the  first  of  the  three 
groups  of  nervous  elements  proper  to  the  visual  appa- 
ratus: they  are  frequently  distinguished  as  the  neuro- 
epithelial layer,  or  the  layer  of  visual  cells. 

The  outer  molecular  layer,  like  the  glomeruli  of  the  ol- 
factory bulb,  may  be  regarded  as  chiefly  made  up  of  the 
interlacement  of  arborizations  and  dendrites,  the  central 


348  PART   II.      HISTOLOGICAL   ANATOMY. 

terminals  of  the  rod  and  cone  elements  here  coming  into 
relation  with  the  peripheral  terminals  of  the  corpuscles  of 
the  inner  nuclear  la^'-er;  or,  as  it  is  now  frequently  desig- 
nated, the  layer  of  bipolar  corpuscles. 

Each  of  these  corpuscles  is  prolonged  peripherally  b\'  a 
filament  which  terminates  at  the  outer  molecular  layer  b\' 
a  group  of  dendritic  processes:  Cajal  has  shown  that 
those  of  some  of  them  pass  the  outer  portions  of  that 
layer  to  form  close  tufts  about  the  knob-like  terminals  of 
the  rod -elements;  the  central  processes  (or  axis-cylinder  pro- 
cesses) of  the  same  corpuscles  passing  to  the  innermost 
portion  of  the  inner  molecular  layer:  for  these  he  has  pro- 
posed the  name  of  rod-bipolars.  The  others  he  has  shown 
to  ramifx'  extensivel}^  in  the  inner  portion  of  the  outer 
molecular  layer,  in  relation  with  the  terminals  of  the 
cone-elements;  while  their  central  processes  terminate  in 
arborizations  w'hich  are  situated  at  various  levels  in  the 
stratified  inner  molecular  laver:  these  he  calls  cone- 
bipolars. 

The  same  investigator  has  demonstrated  in  the  outer- 
most portion  of  the  layer  of  bipolar  corpuscles  elements 
varying"  in  size,  whose  dendrites  ramify  in  the  outer  mole- 
cular layer  and  whose  axis-cylinder  processes  run  for 
longer  or  shorter  distances  horizontally  to  end  in  arbor- 
izations distributed  in  the  same  layer ;  for  which  reason 
he  calls  them  the  horizontal  corpuscles  of  the  retina.  In 
the  innermost  portion  of  the  same  layer  of  corpuscles  he 
has  described  pear-shaped  amacrine  corpuscles  of  vary- 
ing size,  whose  processes  branch  and  ramify  in  the  inner 
molecular  layer  at  various  levels  corresponding  to  those 
indicated  in  connection  with  the  terminals  of  the  cone- 
bipolars.  The  nature  and  functions  of  the  horizontal  and 
amacrine  corpuscles  of  this  layer  may  perhaps  stiil  be  re- 
garded as  matters  of  question ;  the  bipolar  corpuscles 


CHAPTKK    XXIII.    THK  OKCAXS  OF  SriaiAL  SENSE.     349 

clearly  rorin  ihc  second  members   of  tiie  series  ol  nervous 
elements  involved  in  visual  sensation. 

The  inner  molecular  layer  resembles  the  outer  (and,  in- 
deed, all  the  so-called  molecular  layers  of  the  nervous  sys- 
tem) in  consistins^  chieHy  of  an  interlacement  of  central 
and  peripheral  terminal  filaments.  It  shows,  as  has  been 
indicated,  evidence  of  stratification,  due  to  the  termina- 
tion, at  more  or  less  definite  levels,  of  the  central  pro- 
cesses of  the  cone-bipolars,  and  the  associated  horizontal 
distribution  of  their  arborizations ;  in  relation  not  only 
with  the  terminals  of  the  processes  of  the  amacrine  cells, 
but  also  with  those  of  the  dendrites  of  the  corpuscles  of 
the  layer  next  within. 

The  layer  of  ganglion  corpuscles,  the  third  and  in- 
nermost members  of  the  visual  series,  is  composed  of  ele- 
ments which  vary  much  in  size:  according  to  Cajal,  the 
smallest  corpuscles  send  their  dendrites  into  the  inner- 
most stratum  of  the  inner  molecular  layer:  the  largest  to 
the  outermost  stratum :  and  those  of  intermediate  size  in 
like  manner  to  the  intervening  strata ;  the  arborizations 
of  the  terminals  from  the  rod-bipolars  being  distributed 
in  all  cases  in  the  innermost  stratum.  From  each  of  these 
corpuscles  an  axis-cylinder  process  is  given  off  which 
eventually  becomes  one  of  the  fibres  of  the  optic  nerve,  its 
terminal  arborizations  being  situated  in  the  brain.  Cajal 
has  also  described  in  the  optic  nerve  fibres  which  come 
from  the  brain  and  enter  the  retina,  terminatingby  arbor- 
izations within  the  layer  of  bipolar  corpuscles:  he  regards 
them  as  conve^-ing  centrifugal  impulses. 

The  layer  of  bipolar  corpuscles  and  that  of  ganglionic 
corpuscles,  taken  together,  have  been  designated  the  cere- 
bral layer,  as  distinguished  from  the  neuro-epithelial 
layer  or  layer  of  visual  cells.  The  cerebral  and  neuro- 
epithelial layers,  taken  together,  are  formed  from  the  an- 
terior (and  principal)  lamina  of  the  collapsed  optic  vesi- 


350  PART   II.      HISTOLOGICAL   ANATOMY. 

cle,  the  layer  of  pigment  cells  alone  representing  the  pos- 
terior lamina. 

At  the  macula  lutea  the  retinal  layers  are  notably 
thickened,  the  layer  of  ganglionic  coi'puscles  in  particular 
becoming  several  cells  deep.  Passing  towards  the  centre 
of  the  macula,  the  layers  become  rapidl}^  thinned  to  form 
the  fovea  centralis,  in  which  cone-elements  only  are  pre- 
sent in  the  neuro-epithelial  layer,  and  cone-bipolars  in  the 
cerebral  layer,  the  central  processes  of  the  latter  passing 
obliquely  outward  to  enter  the  inner  granular  layer  at 
the  margin  of  the  fovea.  Where  the  optic  nerve  pierces 
the  retina  the  retinal  structures  are  of  course  wanting. 

The  characteristic  retinal  layers  disappear  at  the  ora 
serrata,  the  layer  of  visual  cells  first  becoming  absent. 
Over  the  ciliary  portion  of  the  retina  the  posterior  lamina 
of  the  optic  vesicle  is  represented  by  a  layer  of  pigment 
cells  as  elsewhere:  the  anterior  lamina  bj'  a  layer  of  col- 
umnar cells  l\nng  between  the  pigment  layer  and  the  hya- 
loid membrane:  they  have  large  oval  nuclei  near  their  outer 
extremities.  In  the  iridal  portion  of  the  retina  both  lam- 
inae are  represented  by  layers  of  pigment  cells.  The 
masses  of  retinal  pigment  cells  between  the  ciliary  pro- 
cesses form  the  so-called  ciliary  glands  alread}'' mentioned. 


The  apparatus  of  hearing,  like  that  of  sight,  comprises 
a  receiving  and  a  transmitting  mechanism  in  addition  to 
the  structure  which  contains  the  special  terminal  organs 
involved.  The  receiving  neuro-epithelium  is  in  some  respects 
more  complex  than  in  any  of  the  other  organs  of  special 
sense:  the  accessory  mechanisms  are  far  simpler  than 
those  of  the  apparatus  of  sight. 


CIIAPTHU    XXIII.      Tin:  ORGANS  OF  SIM;CIAI.  SKXSK.      351 

The  pinna  cotiijists  essentially  of  a  sheet  of  yellow  fibro- 
cartilage  covered  by  integument:  funncl-shiiped  or  vari- 
ously modified  in  mammals  generidly,  it  is  in  man  crum- 
pled and  comparatively  rudimentary,  but  retains  its  char- 
acteristic structure.  In  the  lobe  of  the  ear  the  cartilage 
is  rejjUiced  by  a  mass  of  fjit.  The  skin  u]Jon  the  outer  or 
convex  surface  does  not  in  man  differ  materialK'from  that 
of  the  adjacent  portion  of  the  head :  that  of  the  inner  sur- 
face is  thin  and  but  slightly  mobile  upon  the  subjacent 
cartilage,  and  is  devoid  of  sweat  glands.  The  hairs  of  the 
integument  of  the  pinna  are  in  man  very  small,  with  rela- 
tivelv  large  sebaceous  rrlands.  The  small  intrinsic  muscles 
which  pass  from  certain  folds  of  the  cartilage  to  others, 
beneath  the  integument,  are  composed  of  slender  striated 
fibres. 

The  external  auditory  meatus,  in  part  cartilaginous 
and  in  part  bony,  is  lined  by  a  closely  adhering  tegumen- 
tary  layer,  continuous  with  the  skin  upon  the  inner  side 
of  the  pinna,  which  grows  thinner  and  simpler  in  struct- 
ure as  it  passes  inward.  The  portion  of  the  skin  which 
invests  the  surface  of  the  outer  or  cartilaginous  portion  of 
the  tube  contains  fine  hairs  which,  like  those  of  the  pinna, 
iire  accompanied  by  well  developed  sebaceous  glands :  the 
fibrous  tissue  subjacent  to  the  corium  contains  in  addi- 
tion numerous  convoluted  tubular  ceruminous  glands 
which  greatly  resemble  sweat  glands  in  form  and  struct- 
ure but  are  larger  and  more  closely  aggregated  :  they  are 
farther  characterized  by  their  brownish  color  and  the 
highly  refracting  fatty  particles  seen  in  their  secretion. 
The  lining  of  the  deeper  bony  portion  of  the  meatus  is 
devoid  alike  of  hairs  and  of  glands. 

The  membrana  tympani,  which  separates  the  external 
meatus  from  the  middle  ear,  is  composed  of  a  fibrous  lay- 
er invested   outwardly  by  a  continuation   of  the  integ- 


352  PART  II.      HISTOLOGICAL  ANATOMY. 

ument  from  the  bony  wall  of  the  meatus,  and  inwardly 
bv  the  mucous  membrane  which  lines  the  whole  of  the 
tympanic  cavity.  The  fibrous  layer  consists  chiefly  of  ra- 
diating bundles  diverging  chiefly  from  the  point  of  attach- 
ment of  the  malleus:  there  are  in  addition  circularly  dis- 
posed bundles  of  fibres,  chiefl\^  near  the  margin  of  the 
membrane,  which  form  a  so-called  inner  layer.  The  tegu- 
mentarv  la\'er  resembles  that  of  which  it  is  a  continua- 
tion: the  mucous  la\'er  consists  of  a  thin  membrane  rich 
in  elastic  fibres  which  bears  a  single  layer  of  cuboidal  epi- 
thelium whose  component  cells  are  devoid  of  cilia. 

The  auditory  ossicles  possess  to  some  extent  the  char- 
acter of  dense  bone,  their  thicker  portions  showing  dis- 
tinct though  small  Haversian  systems  of  lamellae:  defi- 
nite marrow^  cavities  exist  in  the  interior  of  the  principal 
masses  of  the  malleus  and  incus.  The  articular  surfaces 
are  in  each  case  invested  with  hyaline  cartilage.  The 
muscles  connected  with  the  ossicles  are  composed  of  stri- 
ated fibres.  Both  bones  and  muscles  are  invested  wnth 
the  mucous  membrane  which  lines  the  tympanic  cavity. 

The  Eustachian  tube,  which  connects  the  tympanic 
cavit}^  with  the  pharynx,  has  a  bony  wall  in  its  posterior 
portion;  in  the  anterior  portion  the  wall  is  in  part  com- 
posed of  h\^aline  cartilage,  sparingly  reinforced  by  bund- 
les of  white  fibres,  and  in  part  of  dense  fibrous  membrane. 
It  is  lined  throughout  its  extent  by  a  mucosa  which  is  a 
continuation  of  that  of  the  phar\'nx,  and  is  in  turn  con- 
tinued by  the  lining  of  the  tympanic  cavity:  like  that  of 
the  phar3'nx,  the  mucosa  is  invested  wath  a  layer  of  strat- 
ified columnar  ciliated  epithelium.  In  the  cartilaginous 
])ortion  of  the  tube  there  is  a  submucosa  which  contains 
numerous  mucous  glands  and  a  considerable,  cpiantity  of 
diffuse  adenoid  tissue;  in  this  respect,  again,  recalling  the 
structure  of  the  ph^-rynx.     In  the  bony  posterior  portion 


CHAPTER    XXIII.     THE  ORGANS  OF  SPKCMAL  SENSE.      853 

the  mucosa  is  devoid  of  ;2;laii(ls  and  adheres  more  elosely 
to  the  wall  of  the  tube. 

The  cavity  of  the  tympanum,  like  that  of  the  mastoid 
cells  leadiuij  out  from  it.  may  be  re<^arded  as  an  expan- 
sion of  that  of  the  Eustachian  tube.  It  is  lined  with  a 
mucous  membrane  which  has  already  been  frequentlv  re- 
ferred to :  between  the  membrane  and  the  bony  wall  of 
the  cavity  is  a  submucosa  consisting  of  interlaced  fibrous 
bundles  among  which  are  seen  numerous  spheroidal  bodies 
in  manv  wavs  resem])ling  Pacinian  corjjuscles. 

The  disposition  of  this  fibrous  network,  the  irregulari- 
ties of  the  bony  surfaces  involved,  and  the  structures  pre- 
sent in  the  tympanic  cavity  cooperate  to  throw  the  mu- 
cosa into  cons]3lcuous  folds;  their  disposition  is  a  matter 
for  the  anatomist  rather  than  for  the  histologist.  The 
epithelium  of  the  tympanic  cavity  is  columnar  ciliated 
over  the  greater  ])ortion  of  the  surface;  that  of  the  mas- 
toid cells  is  devoid  of  cilia.  The  existence  of  distinct 
glands  in  the  mucosa  of  the  tympanum  is  a  matter  of 
question. 

The  inner  ear  is  in  the  strict  sense  the  organ  of  hearing, 
the  middle  and  outer  regions  being  merely  accessory 
thereto.  It  consists  of  the  membranous  labyrinth,  in 
which  the  specially  modified  neuro-epithelial  structures  in- 
volved in  audition  are  situated,  enclosed  in  the  cavity 
within  the  periotic  mass  known  as  the  bony  labyrinth. 
The  membranous  labyrinth  is  formed  by  the  ingrowth  of 
the  integument  of  the  side  of  the  head :  this  is  at  first  a 
simple  saccular  or  flask-shaped  cavity  lined  with  epithe- 
lium derived  from  the  ectoderm,  and  communicating  with 
the  surface  by  a  small  aperture.  Later,  this  aperture  is 
obliterated  and  the  sac  is  divided  into  two  principal  re- 
gions, the  utricle  and  the  saccule,  which  are  in  the  adult 
onlvindirectlv  connected.     The  three  semicircular  canals 


354  PART   II.      HISTOLOGICAL    ANATOMY. 

are  developments  of  the  wall  of  the  utricle  and  together 
with  it  form  the  labyrinth  in  the  limited  sense  in  which  the 
term  was  formerly  used.  The  cochlea  is  an  extension  of 
the  saccule. 

The  bon}'  walls  of  the  cavity  which  contains  the  utricle 
and  the  semicircular  canals  are  lined  with  a  thin  perios- 
teum invested  by  flattened  connective  tissue  corpuscles 
which  form  an  endothelium.  The  membranous  structures 
enclosed  within  are  adherent  to  the  periosteum  along  one 
side  of  each  canal  and  upon  a  portion  of  the  surface  of  the 
utricle:  throughout  the  rest  of  their  extent  they  are  sep- 
arated therefrom  by  a  space  filled  with  perihmph  which  is 
traversed  by  frequent  trabeculae  of  fibrous  tissue,  the  free 
surface  being  similarly  invested.  The  membranous  wall 
consists  of  a  layer  of  connective  tissue  containing  numer- 
ous elastic  fibres,  within  which  is  a  dense  clear  tunica 
propria,  whose  surface  throughout  the  canals  shows  nu- 
merous low  papillary  eminences;  lining  the  tunica  is  a 
layer  of  polygonal  pavement-epithelium  cells. 

In  the  ampulla  of  each  of  the  semicircular  canals  the 
tunica  propria  is  much  thickened  along  a  projecting  ridge, 
the  transverse  septum,  upon  whose  summit  is  situated  a 
crista  acustica,  or  ampullar  area  of  auditory  neuro-epi- 
thelium.  In  passing  from  the  general  surface  of  the  am- 
pulla upon  the  sides  of  the  septum  the  pavement  epithe- 
lium becomes  first  cuboidal  and  then  columnar  in  form, 
the  columnar  cells  being  surmounted  by  a  distinct  cutic- 
ular  layer.  Within  the  crista  the  epithelium  consists  of 
cells  of  two  sorts:  fibre  cells,  whose  elongated  bodies  ex- 
tend through  the  whole  epithelium,  their  bases  being 
larger  than  their  free  extremities,  and  their  nuclei  being 
variously  situated  within  the  basal  half;  and  hair-cells, 
cylindrical  elements  which  are  situated  in  the  outer  half 
of  the  epithelium   only,  their  nuclei   being  situated   near 


CIIAPTICK    XXIII.      TFIi:  ORC.AXS  OF  SPKCIAL  SKNSE.      'A3~} 

their  rounded  inner  extremities,  find  their  free  ends  bear- 
ui'fl,  Ion;;  taperin.L!^  lilainonts,  the  auditory  hairs. 

Branches  of  the  auditorv  nerve  are  distributed  to  each 
crista:  as  the  fibres  enter  the  epithelium  they  loose  the 
medullarv  sheath,  and  quickly  divide  into  fibrils  which 
ramify  extensively  in  the  vicinity  of  the  hair-cells,  their 
free  extremities  being  in  every  ease  in  direct  contact  with 
these  ej)ithelial  elements:  the  relation  between  the  nerve 
terminals  and  the  epithelial  cells  must  be  regarded  as  sim- 
ilar to  and  as  specializations  of  that  elsewhere  described 
in  connection  with  the  free  endings  of  nerve  fibre  in  the 
epidermis. 

In  material  hardened  for  section  cutting  the  surface  of 
each  crista  is  found  to  be  covered  by  a  dome-shaped  mass 
of  a  clear  colorless  substance  of  unknown  composition 
and  origin,  in  which  the  auditory  hairs  are  imbedded  :  to 
this  mass  the  term  cupula  is  applied.  Under  suitable 
reagents  the  auditory  hairs  can  be  made  to  break  up  into 
numerous  fine  cilia-like  filaments,  indicating  that  the  hairs 
are  compound  structures. 

The  surface  of  the  utricle  bears  a  large  patch  of  neuro- 
epithelium,  the  macula  acustica  or  macula  cribrosa,  as  it 
is  sometimes  termed,  essentially  similar  in  structure  and 
in  the  mode  of  nerve  supply  to  one  of  the  cristae  of  the 
ampullae.  There  is  not  such  a  marked  thickening  of  the 
subjacent  tunica  propria,  and  the  auditory  hairs  are  not 
as  long  as  those  of  the  ampullar  organs:  the  surface  of 
the  macula  is  invested  b\'  a  soft  gelatinous  mass  in  which 
are  imbedded  numbers  of  crystals  of  calcium  carbonate 
known  as  otoliths.  A  macula  in  every  way  similar  to 
that  of  the  utricle  is  found  in  the  saccule. 

The  cochlea,  a  development  of  a  portion  of  the  saccule, 
with  which  it  is  directly  connected  in  the  lower  verte- 
brates and  in  the  embryo,  is  in  the  mammal  in  great 
measure  constricted  off  from  that  region  in  the  adult,  be- 


856  PART   II.      HISTOLOGICAL   ANATOMY. 

ing  connected  with  it  only  by  a  slender  tubular  passage, 
the  canalis  reuniens.  It  should  be  regarded,  however,  as 
a  tubular  diverticulum  of  that  division  of  the  primary 
auditorv  vesicle,  differing  from  the  rest  of  the  membran- 
ous labyrinth  in  its  spirally  coiled  form,  its  mode  of  at- 
tachment, and  particularly  in  the  complexity  of  its  neuro- 
epithelium,  which  here  attains  a  degree  of  specialization 
found  in  no  other  organ 

Regarding,  for  convenience,  the  position  of  the  whole 
structure  as  so  far  shifted  from  that  which  it  occupies  in 
the  living  body  as  to  bring  the  base  of  the  spiral  into  a 
horizontal  plane,  the  apex  pointing  upward,  the  cochlear 
tube  mav  be  said  to  be  adherent  outwardly  for  about  one 
third  of  its  surface  to  the  bony  wall  of  thecontainingcav- 
itv;  and  to  be  connected  inwardly  with  the  central  bony 
spiral  lamina  b\'  two  flat  membranes,  the  lower  of  which, 
the  basilar  membrane,  is  nearly  horizontal  in  this  posi- 
tion, while  the  upper,  the  membrane  of  Reissner,  slopes 
at  an  angle  of  about  forty-five  degrees:  the  tube  is  there- 
fore approximately  triangular  in  cross  section. 

The  periosteum  of  the  outer  wall  is  much  thickened 
alone:  the  area  of  adhesion  of  the  cochlear  tube  to  form 
the  spiral  ligament,  the  greatest  elevation  being  at  the 
point  of  attachment  of  the  basilar  membrane,  where  a 
fibrous  ridge  is  found  known  as  the  crista  basilaris;  a 
short  distance  above  this  a  second  ridge  is  seen,  the  vas- 
cular prominence,  containing  one  or  more  conspicuous 
bloodvessels :  the  somewhat  concave  surface  between  this 
and  the  ridge  to  which  the  membrane  of  Reissner  is  at- 
tached, known  as  the  stria  vascularis,  exhibits  a  histo- 
logical structure  without  parallel  in  the  entire  body.  It 
contains  a  rich  network  of  capillaries,  imbedded  in  ele- 
ments apparentl}'  epithelial  in  character,  and  commonly 
so  described:  the  superficial  cells  (which  entirely  overlie 
the  capillaries)  are  certainly  in  continuity  with  the  epithe- 


CHAPTKK    XXIII.      THK  ORGANS  Ol-   SI'IXIAI.  SKNSK.      Sf)? 

Hiim  linin«T  the  rest  of  the  tube;  those  situated  between 
the  capillMi'ies  may  very  possibly  l)e  epithelioid  connect- 
ive tissue  corpuscles  not  unlike  those  known  to  occur 
elsewhere. 

The  membrane  of  Reissner  is  an  exceedingly  delicate 
sheet  of  connective  tissue  invested  on  the  side  toward  the 
scala  vestibuli  with  a  layer  of  endothelium  some  of  whose 
cells  are  pigmented  :  the  inner  side  is  lined,  like  the  greater 
portion  of  the  membranous  labyrinth,  by  a  j^avement 
epithelium  composed  of  polyhedral  cells:  the  three  com- 
ponent layers  are  of  nearly  equal  thickness.  The  inner 
and  lower  edge  of  the  membrane  is  united  to  the  middle 
or  inner  portion  of  the  limbus,  a  peculiar  thickening  of 
the  periosteum  of  the  upper  surface  of  the  bon\'  spiral 
lamina. 

The  portion  of  the  limbus  situated  outwardly  from  the 
attachment  of  the  membrane  of  Reissner  terminates  ab- 
ruptly b}'  a  border  excavated  by  the  spiral  groove,  which 
is  bounded  by  an  upper  and  a  lower  lip.  The  upper  sur- 
face of  the  upper  lip  is  ridged  and  grooved  and  its  margin 
developed  into  numerous  tongue-like  processes,  the  audi- 
tory teeth:  the  surface  of  the  ridges  and  the  teeth  is  in- 
vested by  polyhedral  pavement  epithelium  :  that  of  the 
grooves  is  columnar:  this  is  continued  over  the  spiral 
groove  by  a  layer  of  cuboidal  cells  continuous  at  the 
lower  lip  of  the  groove  with  the  epithelial  structures  upon 
the  upper  surface  of  the  basilar  membrane.  The  lower 
lip  extends  to  the  margin  of  the  bony  spiral  lamina. 

The  basilar  membrane  extends  from  this  margin  to  the 
basilar  crest  of  the  spiral  ligament.  Its  middlelayer  consists 
of  a  sheet  of  homogeneous  ground  substance  containing 
scattered  nuclei,  and  having  embedded  in  it  an  immense 
number  of  straight  stout  fibres  running  radially  from  the 
spiral  lamina  to  the  basilar  crest.  The  surface  toward 
the  scala  tympani  is  covered  by  a  layer  of  connective  tis- 


358  PART   II.      HISTOLOGICAL   ANATOMY. 

sue  whose  elements  do  not  take  on  a  definite  endothelial 
form,  but  are  largely  spindle-shaped  fibres  disposed  at 
right  angles  to  the  fibres  of  the  middle  layer.  The  inner 
surface  is  invested  with  epithelium  continuous  with  that 
lining  the  rest  of  the  tube:  that  of  the  outer  half  of  the 
membrane,  or  zona  pectinata,  is  but  slightly  modified  : 
that  of  the  inner  half,  or  zona  tecta,  is  greatly  modified 
to  form  the  characteristic  structure  of  the  cochlea. 

This  structure,  the  organ  of  Corti,  is  a  neuro-epithelium 
not  unlike  those  found  in  the  cristae  and  maculae  acnsticae; 
being  composed,  like  those  bodies,  of  hair  cells  and  sup- 
porting cells:  its  greater  complexity  is  due  chiefly  to  the 
form  and  arrangement  of  the  latter  elements.  The  central 
feature  is  a  series  of  arches  formed  by  the  convergence 
above  of  an  inner  and  an  outer  rod  of  Corti,  the  outer 
rods  being  longer  and  more  slanting  than  the  inner:  the 
triangular  space  beneath  them,  which  runs  the  whole 
length  of  the  cochlea,  beingknown  as  the  tunnel  of  Corti. 
Each  rod  consists  of  a  broad  basal  portion,  or  foot,  a 
slender  shaft,  and  an  enlarged  head,  that  of  the  inner  rod 
having  a  concave  surface  upon  its  outer  side  into  Vt^hich  is 
fitted  a  corresponding  convexity  upon  the  inner  aspect  of 
the  head  of  the  outer  rod :  both  the  inner  and  the  outer 
rods  bear  outwardly  directed  flattened  phalangeal  pro- 
cesses, those  of  the  inner  rods  overlapping  the  inner  por- 
tions of  the  processes  of  the  outer  rods.  Both  inner  and 
outer  rods  are  invested  with  a  layer  of  protoplasm  which 
is  accumulated  at  the  base  on  the  side  toward  the  tunnel 
in  a  mass  containing  an  oval  nucleus.  The  inner  rods  are 
narrower  and  more  numerous  than  the  outer. 

On  the  inner  side  of  the  upper  extremities  of  the  inner 
rods  is  situated  a  row  of  inner  hair  cells,  cylindrical  in 
form,  and,  like  those  of  the  auditory  structures,  only 
extending  through  the  upper  half  of  the  layer:  the  upper 
extremity  of  each  Ijears  a  number  of  hair-like  processes : 


CHAPTER  XXIII.     TIIH  ORGANS  OF  SPECIAL  SKNPE.      359 

the  lower  extremity  is  routuled  and  contains  a  spherical 
nucleus.  Internal  to  the  inner  hair  cells  are  columnar  sup- 
portin«;  cells  which  pass  gradually  over  into  the  epithe- 
lium of  the  spiral  groove. 

On  the  outer  side  of  the  heads  of  the  outer  rods  are  rows, 
three  or  four  in  number,  of  outer  hair  cells,  essentially 
like  those  of  the  inner  row.  Between  them  are  the  ujiper 
extremities  of  the  cells  of  Deiters,  elements  somewhat  re- 
sembling the  rods  of  Corti :  each  has  a  spindle-shaped 
basal  portion  containing  a  spheroidal  nucleus,  and  a  slen- 
der rigid  upper  portion  which  terminates  in  an  outward- 
ly directed  phalanx.  The  phalangeal  processes  of  the  rods 
of  Corti  and  the  phalanges  of  the  cells  of  Deiters  are  united 
by  their  angles  to  form  a  reticular  membrane  through 
whose  apertures  the  extremities  of  the  outer  hair-cells 
project.  Between  the  outer  rods,  the  hair-cells,  and  the 
cells  of  Deiters  are  intervals,  the  spaces  of  Nuel,  which 
communicate  with  each  other  and  with  the  tunnel  of 
Corti,  the   whole  being  filled  with  a  semifluid  substance. 

The  organ  of  Corti  may  be  said  to  be  limited  by  the 
hair  cells  and  the  cells  of  Deiters :  the  latter  pass  over  out- 
wardly into  tall  columnar  elements,  the  cells  of  Hensen, 
whose  nuclei  are  situated  in  their  large  upper  extremities. 
These  pass  rather  abruptl}'  into  the  shorter  columnar 
cells  of  Claudius,  between  which  and  theepitheliumof  the 
outer  wall  of  the  tube  a  gradual  transition  is  seen  along 
the  surface  of  the  zona  pectinata. 

The  bundles  of  nerve  fibres  distributed  along  the  cochlea 
pass  along  the  under  surface  of  the  bony  spiral  lamina 
to  its  margin  from  the  spiral  ganglion  of  the  mod- 
iolus. Here  they  penetrate  the  basilar  membrane :  the 
fibres  loose  their  medullar}'  sheath  and  are  distributed  to 
the  epithelium  in  a  manner  quite  similar  to  that  described 
in  the  account  of  the  cristae,  some  of  the  fibres  travers- 
ing the  tunnel  of  Corti  to  reach  the  vicinity  of  the  outer 


360  PART   II.      HISTOLOGICAL   ANATOMY. 

hair  cells.  From  the  margin  of  the  upper  lipofthelimbus 
a  cuticular  fold,  the  membrana  tectoria,  extends  out  as 
far  as  the  outer  cells.  It  probably  rests  upon  the  organ 
of  Corti  durinof  life. 


A  comparison  of  the  es.sential  structures  of  the  various 
organs  of  special  sense  shows  that  they  agree  in  being 
modified  epithelia  containing  more  or  less  specialized  ter- 
minals of  nervous  elements.  These  epithelia  are  in  [each 
instance  derived  from  the  ectoderm  :  in  the  case  of  the  re- 
tina indirectl}^  the  organ  in  question  being  formed  as  a 
diverticulum  of  the  nervous  axis,  which  is  itself  formed 
from  an  infolding  of  the  ectoderm  :  in  the  case  of  the  other 
sense  organs  the  derivation  from  the  ectoderm  is  direct. 
The  nerve  terminals  of  the  organs  of  taste  and  of  hearing 
resemble  each  other  in  consisting  of  fibrils  ramifying  be- 
tween the  specialized  epithelial  cells,  though  these  senses 
are  not  at  all  related  as  regards  the  character  of  the  stim- 
uli to  which  they  respond.  Similarly,  the  terminals  of  the 
organ  of  smell  and  of  sight  are  somewhat  alike,  greath' 
as  these  senses  differ.  Farther  investigation  may  explain 
these  apparent  resemblances  and  differences  and  exhibit  a 
still  deeper  unity  of  structure  in  the  mechanisms  of  special 
sense. 


INDEX. 


361 


INDEX. 


Pack 

Achromatic  spindle 120 

Achromatin 117 

Adenoid  nodules 180,  183,   271 

Adenoid  tissue,  40, 80,  178,  183,  196 

Adipose  tissue 39 

Adrencils 281 

Adventitia 92,  94,  260,  264 

Agminated  glands 180 

Allantois 257 

Alveoli,  pulmonary 188 

Amacrine  corpuscles 102,  111 

Amitotic  division 119 

Amoeboid  motion 33,  80 

Ampulla 226 

Ampullae,  auditory 354 

Aorta 261 

Apolar  c(irpuscles 110 

Appendix  vermilormis 182 

Arborizations 109,  111 

Areas  of  Colinheim 88 

Areolae 39,  55,  66 

Areolar  tissue 38 

Arteries 91,  92,  259 

Articular  corpuscles 105 

Auditory  meatus 551 

ossicles 362 

teeth 347 

Aucrbach,  plexus  of. 173,  174 

Axilemma 108 

Axis-cylinder 101,  105 

Axis-cylinder  process 102,  110 

Bacillary  Layer 345 

Hellini,  ducts  of 210 

Bertin,  columns  of 205 

Bile  capillaries 189 

Bipolar  corpuscles 102,  109 

Bladder,  urinary   214,257 

Blastoderm 128 


Page 

Blood 14,  73 

corpuscles 74 

platelets 74,  82 

Bone  corpuscles 51 

nutrition  of. 57 

structure  of. 32,  50,  53 

Brain 311 

Bronchi 195 

Bronchial  muscle 196 

Bronchioles 197 

Brunner,  glands  of. 177 

Caecum 182 

Calyces  of  kidney 204,  212 

Canal  of  Schlemm 339 

Canaliculi 49,  51 

Capillaries 91 ,  93 

Capsule  of  Bowman 207 

of  Glisson 186 

of  lens 341 

of  Tenon 336 

Cardiac  fibres 85 

Carotid  glands 280 

Cartilage 25,  29,  30 

calcified 27 

cellular 26 

elastic 27 

hyaline 26 

ossification  in 60 

reticular 27 

transformation  of. 64.  65 

vascularization  of. 63 

Cartilage  bone 61 

Caruncle.... 334 

Cell-division 2.S,  119 

Cells 14,  115 

of  Claudius 359 

colloid 278 

of  Deiters 359 


362 


INDEX. 


Cells  of  Hensen 359 

tactile 103,  104 

Cellular  tissue 38 

Cement  um 164,  166 

Central  canal  of  cord 292,  309 

Centres  of  ossification 61 

Centroacinar  cells 185 

Cerebellar  cortvx 312 

Cerebral  cortex 316 

Ceruminous  glands 144 

Cervix  uteri 242 

Chondrin ^9 

Chondroclasts 56 

Chondrogen 29 

Chordae  tcndinea 263 

Choroid 337 

Chromatin 117 

Chromoplasm 117 

Chromosomes 120 

Ciliary  processes 337 

Circumferential  lamellae 52 

Clitoris 245,258 

Cloaca 257 

Coelom 250 

Coccygeal  gland 280 

Cochlea 355 

Cohnheim,  areas  of. 88 

Collagen 29 

Collaterals Ill 

of  cord 303 

Colloid  cells ■ 278 

Columnae  carnae 263 

Columns  of  Bertin 205 

of  Burdach 298 

Clarke's 296 

of  Goll 298 

of  Morgani 182 

of  Sertoli 221 

of  spinal  cord 292 

of  Tuerck 300. 

Coni  vasciilosi 223 

Conjunctiva 332 

Connective  tissues 24,  32,  38 

Contractile  tissues S3 

Cord,  spermatic 217 

spinal 291 


Corium 138,  155 

Cornea 336 

Corneal  tissue 32,  48 

Corpora  cavernosa 228 

Corpus  Arantii 263 

Highmori 219,  223 

luteum 237 

spongiosum 229 

Corpuscles 14,  23 

accessory 36 

adventitious 36 

amacrine 102,  111 

apolar 110 

articular 105 

basket 315 

bipolar 102,  109 

blood , 73 

bone 51 

of  Cajal 316. 

colored .      74 

colorless 74,  77 

concentric 278 

. corneal 49 

fixed 31,  36 

genital 105,  230.  244 

of  Gran  dry 103 

, of  Hassali 278 

Malpighian 205.  276 

. of  Martinotti 318 

of  Melssner 104 

— —migratory 33,  36 

mitral  327 

multipolar 102,  110 

of  muscle 87 

nerve 102,  111 

nerve-fibre 101 

of  Purkinje 313 

pyramidal 317 

red  74 

of  retina 348 

of  spinal  cord 305 

tactile 104 

unipolar.... 102.  110 

of  Vater 105 

wliite 74 

Cowpcr,  glands  of 231 


INDEX. 


363 


Crcmaster 218 

Crescents  of  Ciianuzzi 158 

Crista  acustica 354 

Crtista  i>etrosa IfiG 

Crypts  of  Lielirrkuhn 177 

Crystalline  letis 340 

Cumulus  proligerus 236 

Cupula 355 

Cytology 15 

Daktos 218 

DeBove.  endothelium  of. 178 

Deiters,  process  of. 102 

Demilunes  of  Heidenhain 159 

Dendrites 102,  109,  110 

Dense  bone 52,  69 

f«)rmation  of. 64 

Dental  papilia 167 

Dentinal  tubules 165 

Dentine ..32,  48.  164 

Dcutoplasm 116 

Discharging  terminals 106- 

Discus  proligerus 236 

Disks,  tactile 104 

Division,  amitotic 119 

karyokinetic 119 

Dobie's  line 88 

Dorsal  coniua 292 

Duct  of  Gartner 240 

Ductless  bodies 269 

Duodenum 177 

Dura 289 

E.\R 350 

Ebncr,  glands  of. 170 

Ectodei-m 128 

Ectoplasm 116 

Elastic  cartilage 27 

fibres 34 

membrane 41 

tissue 43 

Elastin 35 

Elementary  particles 82 

Enamel 164,  166 

germ  167 


End-bulbs  of  Krausc 104 

Endings,  free  nerve 103 

Endocardium 262 

Endochondral  bone 64,  67 

Endomysium 90 

Endoneurium 112 

Endoplasm HG 

Endosteum 59 

Endothelium 20,  44 

of  DeBove 178 

End-plates,  motor 106 

Entoderm 128 

Eosinophile  cells..... 37 

leucocytes 79 

Ependyma 309 

Epiblast 128 

Epicardium 262 

Epidermis ,. 135 

Epididymis 217,  223 

Epineurium 112 

Epiphysis  cerebri 284 

Epithelium 17 

ciliated.... 19 

columnar 8l 

germinal 234 

— —  glandular IS 

pavement 18 

polyhcdrai 18 

respiratory 198 

simple 19 

spheroidal .^.  19 

squamous 18 

stratified 19 

transitional 19,  214 

Epoophoron  239 

Erectile  tissue 228 

Erectorcs  fili 15o 

Erythroblasts .'i5,  81 

Erythrocytes 74 

Eyelids 331 

Factors  of  strictire 14 

Fallopian  tubes 238 

Fasciculi,  muscular 89 

Fat  cells ....  37 


364 


INDEX. 


Fenestrated  membrane 42 

Ferrein,  pyramids  of. 205 

Fertilized  ovum 86 

Fibres 34 

elastic 34 

gray 101.  107 

medullated 101 

of  Mueller 344 

non-meduUated 101 

of  Purkinjc 563 

of  Remak lOl 

smooth  muscular 84 

striated 87 

of  Tomes 165 

white 34,  101,  107 

Fibrillae,  muscular 88 

•  primitive 107 

Fibrin 73 

Fibro-cartilage 27 

Fibrosa 173,  194,  196,  216,  244 

Fibrous  membrane 41 

tissues 31,  32,  42,  45 

Fields  of  Cohnheim 88 

Fixed  corpuscles 31,  36 

Follicles 157 

intestinal 177 

Foveolae 56 

Free  nerve  endings 103 

Funiculus 112 

Ganglia 113 

Ganglionic  columns 294 

Gartner,  duct  of. 240 

Gelatin 29 

Gelatinous  fibres 101 

tissue 32 

Genital  corpuscles 105,  230,  244 

eminence 257 

ridges 257 

Germinal  epithelium 234 

ridge 255 

spot 237 

vesicle 237 

Gianuzzi,  crescents  of. 159 

Giant  cells 55 

Giraldes,  organ  of 223 


Glands 157 

of  Bartholin 245 

of  Brunner 177 

cardiac 175 

carotid 280 

ceruminous 144,  351 

coccygeal 280 

of      owper 231 

of  Ebner 170 

gastric 175 

lachrymal 334 

of  Littre 227 

mammarj' 246 

Meibomian 145,  333 

-  of  Moll 332 

of  mouth 160 

mucous 158 

peptic 175 

prostate 231 

pyloric 175 

salivary 161 

sebaceous 144 

serous 158 

of  skin 142 

sudoriparous 152 

of  Tyson 230 

unicellular 156 

uterine 240 

Glandular  epithelium 18 

Glans  penis 230 

Glia-cells 46,113 

Glisson,  capsule  of. 186 

Glomeruli,  renal 207 

olfactorj' 327 

Goblet  cells 19.  156 

Golgi,  organs  of. 103 

Gonads 217 

Graafian  follicles 234 

Grandry,  corpuscles  of. 104 

Granule  cells 37 

Gray  commissure 292 

fibres 101,  107 

matter  of  cord 292 

Gums 154 

Haematoblasts 82 


INDEX. 


365 


Haemoglobin 76 

Hairs U6 

Hassall's  corpuscles 27H 

Haversian  canals 52 

spaces ".  53 

systems .- 52.  166 

Head  kidney 252 

Heart 261 

Heidenhain,  demilunes  of. 159 

Henle's  layer l+H 

loop 208 

sheatli 113 

He|)atic  cords 188 

Horny  layer 135 

Houston,  valves  of. 182 

Howship's  lacunae 56 

Huxley's  layer 148 

Hyaline  cartilage 26 

Hyaloid  menibrane 342 

Hyaloplasm 1 15 

Hydatids  of  Morgagni 223.  224 

stalked 239 

Hymen 244 

Hypoblast 128 

Hypophysis  cerebri 283 

I.NCREMENTAL  LINKS  OF  SALTER,  165 

Infundibula,  pulmonary 197 

Intima 92,  259,  264 

Interglobular  spaces 165 

Interlobular  cells  of  pancreas..  185 

Internal  sphincter 184 

Internodes 101 

Interstitial  lamellae 52 

Intestinal  follicles 177 

Intramembranous  ossification, 60, 61 

Involuntarv  muscular  tissue 84 


Jelly  of  Wharto.n 


33 


Karvokinksis 119 

Kidneys 203 

bloodsupply  of : 205 

permanent 252 

primitive 252 


Krausc,  end-bulbs  of 104 

membrane  of 88 

Laijia  majora 246.  257 

minora 245.  257 

Lachrymal  glands 334 

Lacteal  glands 267 

Lacunae,  of  bone 49,  50 

Howship's 56 

of  Morgagni  227 

Lamellae,  circumferential 52 

interstitial. 52 

Ivameilated  tissues 47 

Lamina  cribrosa 335 

fusca 335 

-  -  suprachoroidea 337 

Large  intestine 181 

Larynx 199 

Lateral  cornu .-  292 

Leucocytes 33,  36,  74.  77 

Lieberkuhn,  crypts  of. 177 

Lines  of  Schreger 165 

Lingual  papillae 169 

Littre,  glands  of. 227 

Liver 185 

Lymph  96 

capillaries 92,  96 

vessels 92,97,  264 

Lymphatic  glands 80,  272 

Lymphobiasts 81 

Lymphocytes 80 

Lymphoid  tissue 80 

Macula  acistica 355 

Malpighian  corpuscles 205,  276 

Malpighi,  lacunae  of  227 

pyramids  of 204 

Mammary  glands 246 

Marrow 40,  53,  54.  81 

Matrix  23 

Media 92,  259.  264 

Mediastinum  testis 219 

Medullated  fibres 101.  Iu6 

Medullary  sheath lOl 

Meibomian  glands 144 

Meissner,  corpuscles  of. 104 


366 


INDEX. 


Meissner, plexus  of  172, 174, 178, 183 

Melanin 37 

Membrana  granulosa 236 

propria 41 

tectoria 360 

Membrane,  basilar 356 

bone 61 

of  Bowman 336 

of  Descemtt 336 

of  Krause 88 

Nasm\'tirs 166 

periodontal 166 

serous 98 

tympanic 351 

Membranous  labyrinth 353 

Meninges 267,  288 

Menisci,  tactile 104 

Merkel,  tactile  cells  of  103 

Mesenchyma 130 

Mesentery  267 

Mesoblast 128 

Mesoderm 128 

Mesogaster 177 

Mesonepliros 252,  254 

Metanephros 252 

Microcytes 75 

Migratory  corpuscles 33,  36 

Mitosis 119 

Mons  veneris 246 

Morgagni,  columns  of. 182 

hydatids  of 223.  224 

Motor  end-plate   106 

Mucosa, 154,  155.  172,  175,  177, 

181,  182,  192,  215,238,240,243 

Mucous  glands 158 

layer  of  skin 135 

membranes 153 

tissue 32 

Muellerian  duct 253 

Multipolar  corpuscles 102,  110 

Muscle 89 

bronchial 196 

corpuscles H7 

papillary 263 

of  Kiolan 332 

tracheal 194 


Muscular  tissues 84 

cardiac 36 

smooth 84 

striated 37 

Muscularis  mucosa. ...lt>6,  172,  174, 

176,  178,  183,  241. 
Musculosa,  172,  174.  177,  llsi,  184, 

216.  239,  241,  244. 

Myelin  101 

Myeloplaxes 55 

Myocardium 262 

Nails 15 1 

Nasmyth's  membrane 166 

Nephridium 249 

Nephrostome 250 

Nerve  corpuscles 102,  111 

fibres 101,  109 

fibre  corpuscles 101 

terminals 102 

Nerves 112 

Nervous  tissues 99 

Neurilemma 101,  109 

Neuro-epithelium 222,  325,  347, 

354,  308,  360. 

Neuroglia 46,  113 

Neurokeratin  108 

Neuroplasm  107 

Nodes  of  Ranvier  101 

Nodules,  adenoid IHO,  183 

Non-meduUated  fibres 101,  105 

Nuclear  division 118 

Nucleoli -   117 

Nucleus 16,  110 

Odontoblasts 164 

Oesophagus 174 

Olfactory  bulbs 325 

cells  325 

glomeruli 327 

hairs 325 

Omentum 177,  267 

Oosperm  128 

Organ 14,  127 

of  Corti 358 

of  Ciraldcs  223 


INDEX. 


367 


Orjjf.'iii  of  (iolgi 103 

of  Roseuniucllcr 2.'}9 

Os  uteri 24-:{ 

Osier's  jj;ra miles  82 

Ossein 51 

Osseous  tissue 50 

Ossification 59 

centres  of (il 

in  cartila^'c 60.  63 

in  membrane 60,  61 

Osteo!)lasts 5+.  59 

Osteoclasts  56 

Osteogenetic  fibres 62 

-  layer 54,59 

tissue 66 

Otoliths 355 

Ova,  primitive 234- 

primordial 255 

Ovary 233 

Oviducts 238,  253 

Ovula  Nabotlii 242 

Ovum 128,  236 

I'aciman  noDiKs 105,  230 

Palate 154 

Pancreas 184 

Pannicuius  adiposus 140 

Pa])i!lae,  foliate 322 

of  kidney 203 

of  Iin<,nial 169 

Papillary  muscles 263 

Parablast 130 

Paradidymis 225 

Paraplasm    116 

Paratliyroids 279 

I'arietal  cells 176 

Paroophoron 240 

Parovarium 239 

Pavement  epithelium 18 

Pelvis  of  kidney  212 

Penis 227,  257 

Peptic  cells 176 

Pericardium 266 

Perichondral  bone 64.  69 

Perichondrium 30 


Periodontal  membrane 166 

Perimysium 90 

Perineurium 112 

Periosteum  53,  54.  66,  68 

Peritoneum 266 

Perivascular  lymphatics..  98 

Permanent  marrow 67 

Peyer's  patches 180 

Pflue^er,  tubes  of. 161 

Pharynx 173 

Pia 289 

Pigment  cells 37 

Pineal  body 284 

Pituitary  body 283 

Plain  muscular  tissue 84 

Plasma  of  blood... 73 

cells 37 

Platelets,  blood 74,  82 

Pleurae 266 

Plexus  of  Auerbach 173,  174,  177 

of  Meissner..l72,  174,  178,  183 

Plica  semilunaris 333 

Poles  of  nerve  corpuscles 102 

Polyhedral  epithelium 18 

Portal  canals 186 

Prickle  cells 19 

Primary  areolae 65,  66 

marrow 67 

vascular  invasion 67 

F'rimitivc  fibrillae 107 

ova 234 

sheath 101 

Process,  axis  cylinder 102 

of  Deiters 102 

Pronephros 252 

Prostate  gland 231 

Protoplasm 15,  115 

Protoplasmic  processes 102,  110 

Pulmonary  artery 261 

Pupil,  muscles  of 339 

Pylorus 1  79 

Pyramids  of  Ferrein 205 

of  Malpighi 204 

Pyriform  corpuscles 101,  109 


368 


INDEX. 


Ranvier,  nodes  of 101 

Receiving  terminals 102 

Rectum 182 

Red  corpuscles 74 

marrow 55 

Remak,  fibres  of 100 

Rete  mucosum 135 

testis 220 

Reticular  cartilage 27 

Reticulum 1 15,  121 

Retiform  tissue 4-0 

Retina 344 

Retinal  pigment 346 

Retzius.  stripes  of. 106 

Ring-muscle 338 

Root-sheatli  of  hair 147 

Saliva 163 

Salivary  glands 161 

Salter,  incremental  lines  of. 165 

Sarcolemma 87 

Sarcoplasm 88 

Sarcous  elements 88 

Schachowa,  spiral  tubule  of 208 

Schmidt,  medullary  segments  of  108 

Schreger,  lines  of. 165 

Schwann,  white  substance  of....   101 

Sclerotic 335 

Scrotum  217,  258 

Sebaceous  glands 144 

Secondar3'  areolae 65,  66 

Segmental  ducts 252 

organs 250 

Segmentation 128 

Seminal  vesicles  226 

Seminiferous  tubules 220 

Serosa 173,  177,  181,  239 

Serous  endothelium 20,  98 

glands 158,  159 

membranes 99,  265 

Sertoli,  columns  of 221 

Sharpey's  fibres 53 

Sheath,  Henle's 113 

Simple  epithelium 19 

Sinus  pocularis 231,  232 


Skeletal  tissues 45 

Skin 135,  141,  217,  230,  331,  351 

Small  intestine 179 

Small  vessels 91 

Smooth  muscular  fibres 84 

Sole-plate 106 

Solitary  follicles 170 

Spaces  of  Fontana 339 

interglobular 165 

Spermatic  cord 217 

Spermatozoa 220,  222 

Spermiduct 254 

Spheroidal  epithelium 18 

Spinal  cord 291 

ganglia 309 

nerves 301 

Spleen 274 

Spirem • 119 

Spongioplasm  115 

Spongy  bone- 52 

Squamous  epithelium 18 

Stalked  hydatid 239 

Stellate  corpuscles 113 

Stellules  of  Verhuyen 206 

Stigmata 94 

Stomach 175 

Stomata 98,  198 

Stratified  epithelium 19 

Stratum  adiposum 140 

corneum 137 

epitrichium 137 

granulosum 136 

lucidum  137,  152 

Malpighii 136,  152 

papillare 139 

reticulare 139 

squamosum I'SH 

Stria  vascularis 356 

Striated  muscular  fibres 87 

Striped  muscular  fibres  ...   87 

Stripes  of  Retzius 166 

Subcutanea 140 

Submucosa 154,  156,  172,  174 

177,  178,  181,  183,  193,  196,  216 
239,  241,  244. 


INDEX. 


369 


Substance  of  Rolando Ii95.  307 

Sudoriparous  slj^nds 142 

Su|)rarciial  i.'a|)Siiles....j 2Ml 

Svmpatlictic  system 110 

Synovial  nienihrancs 268 

Tactilk  cklls 103,  lO-l- 

cor|)usclcs 104- 

disks 104 

hairs 150 

Taste-buds 322 

Teeth 16+ 

Tendon  cells 43 

tissue 42 

Terminals,  discharf^in^ 106 

nerve 102 

receiving 10  i 

Testis 217,  219 

Theca  folliculi 235 

Thoracic  duct 265 

Thymus 277 

Thyroid 27S 

Tissue 13 

Tomes,  fibres  of. 165 

Tongue 168 

Tonsils 171,  173 

Trabeculae  of  spongy  bone 53 

Trachea 192 

Tracheal  muscle 194 

Tracts  of  cord 299 

Transitional  epithelium 19 

Tubes  of  Ptlueger 161 

Tunica  albuginea 219,  229,  234 

Tympanum 353 

I'mbilical  cord 33 

Uniceilular  glands 156 

Unipolar  coipuscles 102,  110 

Ureter 213 

Urethra,  female 216 

male 226 

Urinary  bladder 214,  257 

Uriniferous  tubules 207 

Urogenital  sinus 257 


U'tcrine  glands 240 

Uterus 240 

masculinus 231 

Vagina 243 

Valves  ot  heart 263 

of  Houston 182 

of  veins 264 

Vas  aberrans 223,  224 

deferens 219,  224,  225 

Vasa  cfferentia 223 

vasorum 261 

Vascular  endothelium .20,  90 

Vascularization  of  cartilage 63 

Vatcr,  corpuscies  of. 105 

Veins 91,  94,  263 

I    Ventral  cornu 292 

I    Verhuyen,  stellules  of. 206 

Vermiform  appendix 182 

Vessels 91 

Vestibule 245 

Villi 177 

Vitelline  membrane 237 

Vitellus 237 

Vitreous  body 33,341 

Volkmann's  canals 52 

Vulva 244 

Wharto.n's  jelly 33 

White  commissure 293 

fibres 34,  lOl,  lo7 

fibrocartilage 28 

fibrous  tissue 42 

matter  of  cord 292 

substance  of  Schwann 101 

Wolffian  body ...  253 

duct 253 

Yellow  fibres 34 

fibro-cartilage 27 

marrow 56 

Zona  pellccida 236 

striata 236 

vasculosa 234 

Zone  of  Zinn 343 


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